diff --git a/aeson-flowtyped.cabal b/aeson-flowtyped.cabal
--- a/aeson-flowtyped.cabal
+++ b/aeson-flowtyped.cabal
@@ -1,5 +1,5 @@
 name:           aeson-flowtyped
-version:        0.12.0
+version:        0.12.1
 synopsis:       Create Flow or TypeScript type definitions from Haskell data types.
 description:    Create Flow or TypeScript type definitions from Haskell data types.
 category:       Web
diff --git a/src/Data/Aeson/Flow.hs b/src/Data/Aeson/Flow.hs
--- a/src/Data/Aeson/Flow.hs
+++ b/src/Data/Aeson/Flow.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE ConstraintKinds           #-}
 {-# LANGUAGE CPP                       #-}
 {-# LANGUAGE DataKinds                 #-}
 {-# LANGUAGE DefaultSignatures         #-}