incorrect type error message

Consider

module Bug where

e = fmap id 1

ghci --version
The Glorious Glasgow Haskell Compilation System, version 5.02.2

says

:l Bug.hs 
Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:5:
    No instance for (Num (f a))
    arising from the literal `1' at Bug.hs:5
    In the second argument of `fmap', namely `1'
    In the definition of `e': fmap id 1

Bug.hs:5:
    Ambiguous type variable(s) `f' in the constraint `Functor f'
    arising from use of `fmap' at Bug.hs:5
    In the definition of `e': fmap id 1

although

:t (fmap id 1)
forall f a. (Num (f a), Functor f) => f a

Clearly, 'f' is not ambiguous.

Martin