diff --git a/src/Deriving/OpenApi.hs b/src/Deriving/OpenApi.hs
index c75645d..8e61f36 100644
--- a/src/Deriving/OpenApi.hs
+++ b/src/Deriving/OpenApi.hs
@@ -72,6 +72,7 @@ instance OpenApiOptions xs => OpenApiOptions (OmitNothingFields ': xs) where
 
 instance OpenApiOptions xs => OpenApiOptions (RejectUnknownFields ': xs) where
   openApiOptions = openApiOptions @xs
+  openApiSchemaModifier = schema . additionalProperties .~ Just (AdditionalPropertiesAllowed False)
 
 instance (StringModifier f, OpenApiOptions xs) => OpenApiOptions (FieldLabelModifier f ': xs) where
   openApiOptions = (openApiOptions @xs) {fieldLabelModifier = getStringModifier @f}
diff --git a/stack.yaml b/stack.yaml
index b052106..22f42ee 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,4 +1,4 @@
-resolver: lts-16.26
+resolver: lts-18.21
 
 packages:
   - .