instance not found

Hello,

I have the following code:
    class C a where
        f :: a -> Int

    instance C (a,b) where
        f = const 2

    instance C ((a,b),c) where
        f = const 3

If I load this code into ghci with switches -fglasgow-exts and
-fallow-overlapping-instances and enter f (1,2), I get this message:
    <interactive>:1:
        No instance for (C (t, t1))
        arising from use of `f' at <interactive>:1
        In the definition of `it': f (1, 2)

I cannot see why f (1,2) shouldn't be acceptable, so I suppose it's a bug. Am 
I missing something?

I'm using GHC 5.04.3.

Wolfgang