diff --git a/purescript.cabal b/purescript.cabal
--- a/purescript.cabal
+++ b/purescript.cabal
@@ -1,5 +1,5 @@
 name: purescript
-version: 0.4.5
+version: 0.4.5.1
 cabal-version: >=1.8
 build-type: Custom
 license: MIT
diff --git a/src/Language/PureScript/TypeChecker/Types.hs b/src/Language/PureScript/TypeChecker/Types.hs
--- a/src/Language/PureScript/TypeChecker/Types.hs
+++ b/src/Language/PureScript/TypeChecker/Types.hs
@@ -583,8 +583,9 @@
   env <- getEnv
   case M.lookup ctor (dataConstructors env) of
     Just (_, ty) -> do
-      (_, fn) <- instantiatePolyTypeWithUnknowns (error "Data constructor types cannot contains constraints") ty
-      go binders fn
+      (_, fn) <- instantiatePolyTypeWithUnknowns (error "Data constructor types cannot contain constraints") ty
+      fn' <- replaceAllTypeSynonyms fn
+      go binders fn'
         where
         go [] ty' = do
           _ <- subsumes Nothing val ty'