Rank-2 polymorphism and pattern matching

The following won't compile for me isnull :: (forall a . [a]) -> Bool isnull ([] :: forall b . [b]) = True Couldn't match expected type `forall b. [b]' against inferred type `[a]' In the pattern: [] Wrapping it in a constructor doesn't help, though I can define a "null": data NullList = NullList (forall a . [a]) null = NullList [] isnull2 :: NullList -> Bool isnull2 (NullList []) = True Why? Jim