diff --git a/test/golden/additional-properties.yaml.out b/test/golden/additional-properties.yaml.out
index 846984e..cdf08f4 100644
--- a/test/golden/additional-properties.yaml.out
+++ b/test/golden/additional-properties.yaml.out
@@ -278,13 +278,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -304,11 +304,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/bug-1.yaml.out b/test/golden/bug-1.yaml.out
index 9861381..e48fdcb 100644
--- a/test/golden/bug-1.yaml.out
+++ b/test/golden/bug-1.yaml.out
@@ -264,13 +264,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -290,11 +290,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/bug-2.yaml.out b/test/golden/bug-2.yaml.out
index 8746c50..ac77ad5 100644
--- a/test/golden/bug-2.yaml.out
+++ b/test/golden/bug-2.yaml.out
@@ -264,13 +264,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -290,11 +290,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/csv.yaml.out b/test/golden/csv.yaml.out
index 3383681..64ea806 100644
--- a/test/golden/csv.yaml.out
+++ b/test/golden/csv.yaml.out
@@ -278,13 +278,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -304,11 +304,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/datetime.yaml.out b/test/golden/datetime.yaml.out
index fe1ba4f..f9c4ad9 100644
--- a/test/golden/datetime.yaml.out
+++ b/test/golden/datetime.yaml.out
@@ -264,13 +264,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -290,11 +290,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/enum-bug.yaml.out b/test/golden/enum-bug.yaml.out
index 113bc02..f1c32a2 100644
--- a/test/golden/enum-bug.yaml.out
+++ b/test/golden/enum-bug.yaml.out
@@ -267,13 +267,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -293,11 +293,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/enum.yaml.out b/test/golden/enum.yaml.out
index c7a480c..45baa53 100644
--- a/test/golden/enum.yaml.out
+++ b/test/golden/enum.yaml.out
@@ -331,13 +331,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -357,11 +357,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/haskell-ext.yaml.out b/test/golden/haskell-ext.yaml.out
index 20b0064..0b30cc7 100644
--- a/test/golden/haskell-ext.yaml.out
+++ b/test/golden/haskell-ext.yaml.out
@@ -282,13 +282,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -308,11 +308,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/headers.yaml.out b/test/golden/headers.yaml.out
index 06fed75..189635a 100644
--- a/test/golden/headers.yaml.out
+++ b/test/golden/headers.yaml.out
@@ -295,13 +295,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -321,11 +321,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/lists.yaml.out b/test/golden/lists.yaml.out
index f3ba781..5363c1c 100644
--- a/test/golden/lists.yaml.out
+++ b/test/golden/lists.yaml.out
@@ -328,13 +328,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -354,11 +354,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/numbers.yaml.out b/test/golden/numbers.yaml.out
index 53ae705..7ff0d13 100644
--- a/test/golden/numbers.yaml.out
+++ b/test/golden/numbers.yaml.out
@@ -264,13 +264,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -290,11 +290,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/object-without-type.yaml.out b/test/golden/object-without-type.yaml.out
index d2ec9c6..a61a7bf 100644
--- a/test/golden/object-without-type.yaml.out
+++ b/test/golden/object-without-type.yaml.out
@@ -264,13 +264,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -290,11 +290,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/oneof.yaml.out b/test/golden/oneof.yaml.out
index 1f2c586..6f5ecd9 100644
--- a/test/golden/oneof.yaml.out
+++ b/test/golden/oneof.yaml.out
@@ -294,13 +294,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -320,11 +320,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/petstore.yaml.out b/test/golden/petstore.yaml.out
index 12713f1..80facee 100644
--- a/test/golden/petstore.yaml.out
+++ b/test/golden/petstore.yaml.out
@@ -295,13 +295,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -321,11 +321,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}
diff --git a/test/golden/test1.yaml.out b/test/golden/test1.yaml.out
index d788c71..5f3e8e1 100644
--- a/test/golden/test1.yaml.out
+++ b/test/golden/test1.yaml.out
@@ -298,13 +298,13 @@ requiredQueryParameter ::
 requiredQueryParameter name withParam = \request respond ->
   case List.lookup name (Wai.queryString request) of
     Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just Nothing ->
-      respond (Wai.responseBuilder (toEnum 400) [] mempty)
+      respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam x request respond
 {-# INLINEABLE requiredQueryParameter #-}
@@ -324,11 +324,11 @@ optionalQueryParameter name allowEmpty withParam = \request respond ->
       | allowEmpty ->
           withParam Nothing request respond
       | otherwise ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Missing query parameter: " <> name))
     Just (Just value) ->
       case parseQueryParam (Text.decodeUtf8 value) of
         Left _err ->
-          respond (Wai.responseBuilder (toEnum 400) [] mempty)
+          respond (Wai.responseBuilder (toEnum 400) [] ("Unable to recognize query parameter: " <> name))
         Right x ->
           withParam (Just x) request respond
 {-# INLINEABLE optionalQueryParameter #-}