diff --git a/src/Tie/Codegen/Operation.hs b/src/Tie/Codegen/Operation.hs
index a4cb1f0..5212f39 100644
--- a/src/Tie/Codegen/Operation.hs
+++ b/src/Tie/Codegen/Operation.hs
@@ -331,12 +331,13 @@ codegenQueryParamGuard Param {name, required} continuation
               )
         )
   | otherwise =
-    "case" <+> "fmap" <+> "(" <> "fmap" <+> "(" <> "Web.HttpApiData.parseUrlPiece" <+> "." <+> "Data.Text.Encoding.decodeUtf8" <> ")" <> ")" <+> "("
+    "case" <+> "Control.Monad.join" <+> "(" <> "fmap" <+> "(" <> "fmap" <+> "(" <> "Web.HttpApiData.parseUrlPiece" <+> "." <+> "Data.Text.Encoding.decodeUtf8" <> ")" <> ")" <+> "("
       <> "Data.List.lookup" <+> "\""
       <> toParamName name
       <> "\"" <+> "("
       <> "Network.Wai.queryString" <+> "request"
       <> ")"
+      <> ")"
       <> ")" <+> "of"
       <> PP.line
       <> PP.indent
diff --git a/test/golden/test1.yaml.out b/test/golden/test1.yaml.out
index 938e624..33b8a2a 100644
--- a/test/golden/test1.yaml.out
+++ b/test/golden/test1.yaml.out
@@ -77,7 +77,7 @@ application run api notFound request respond =
                                         Just (Left err) ->
                                             undefined
                                         Just (Right page) ->
-                                            case fmap (fmap (Web.HttpApiData.parseUrlPiece . Data.Text.Encoding.decodeUtf8)) (Data.List.lookup "offset" (Network.Wai.queryString request)) of
+                                            case Control.Monad.join (fmap (fmap (Web.HttpApiData.parseUrlPiece . Data.Text.Encoding.decodeUtf8)) (Data.List.lookup "offset" (Network.Wai.queryString request))) of
                                                 Just (Left err) ->
                                                     undefined
                                                 _x ->
@@ -99,7 +99,7 @@ application run api notFound request respond =
                                         Just (Left err) ->
                                             undefined
                                         Just (Right page) ->
-                                            case fmap (fmap (Web.HttpApiData.parseUrlPiece . Data.Text.Encoding.decodeUtf8)) (Data.List.lookup "offset" (Network.Wai.queryString request)) of
+                                            case Control.Monad.join (fmap (fmap (Web.HttpApiData.parseUrlPiece . Data.Text.Encoding.decodeUtf8)) (Data.List.lookup "offset" (Network.Wai.queryString request))) of
                                                 Just (Left err) ->
                                                     undefined
                                                 _x ->