[Haskell-beginners] Why doesn't ghci load this?

Hi, I've been making my way through the exercises in the book "Real World Haskell". Below is my solution to an exercise asking for an implementation of groupBy in term sof a foldr. I haven't tested it extensively but is seems to work. GHCI rejects it though, if I uncomment the type signature for the 'step' function. Could someone please help me to understand why that is? Thanks, Jeff myGroupBy :: (a -> a -> Bool) -> [a] -> [[a]] myGroupBy p xs = foldr step [[]] xs where -- step :: a -> [ [ a ] ] -> [ [ a ] ] step x acc@( ys : yss ) | null ys = [ x ] : [] | p x ( head ys ) = ( x : ys ) : yss | ( p x ( head ys ) ) == False = [ x ] : acc