[Haskell] Ambiguity problem

Hi I am struggling with an ambiguity problem. I have the following code:

foo :: Constr -> Int foo = numChildren . fromConstr

numChildren :: (Data a) => a -> Int numChildren x = sum $ gmapQ (\_ -> 1) x

which I thought would work, but the GHC-6.4 says:

Ambiguous type variable `a' in the constraint: `Data a' arising from use of `numChildren' at AutoForm.hs:87:6-16 Probable fix: add a type signature that fixes these type variable(s)

I cannot see why I need to add (Data a), as fromConstr signatur is: fromConstr :: forall a. (Data a) => Constr -> a I do not know where to add it either. Hope, somebody can enlighten me. Greetings, Mads Lindstrøm