diff --git a/css-syntax.cabal b/css-syntax.cabal
--- a/css-syntax.cabal
+++ b/css-syntax.cabal
@@ -1,5 +1,5 @@
 name:                css-syntax
-version:             0.0.1
+version:             0.0.2
 
 synopsis: This package implments a parser for the CSS syntax
 description:
diff --git a/src/Data/CSS/Syntax/Tokens.hs b/src/Data/CSS/Syntax/Tokens.hs
--- a/src/Data/CSS/Syntax/Tokens.hs
+++ b/src/Data/CSS/Syntax/Tokens.hs
@@ -455,7 +455,7 @@
     | otherwise              = False
 
 
-unhex :: Monad m => String -> m Int
+unhex :: (Functor m, Monad m) => String -> m Int
 unhex = fmap toInt . go []
   where