`String' is applied to too many type arguments

I'm just learning Haskell and I was trying to write a simple program to eliminate the first n characters from a String. This is what I got:

cutString :: (Num n, String str) => n -> str -> str

cutString n str = case n of
        0 -> tail str
        n -> cutString (n-1) (tail str)

GHC gives me this error though, and I can't figure out why:

`String' is applied to too many type arguments
 In the type signature for `cutString':
 cutString :: (Num n, String str) => n -> str -> str