Error message "Expecting a function type, but found `w_a1Kh'"

When compiling with GHC or loading into GHCi the following module I get a strange error:

module Bla where

bla :: String -> String
bla x = unwords $ [ ('#':) ]

The error is:

Bug.hs:4:20:
    Expecting a function type, but found `w_a1bp'
      Expected type: String
      Inferred type: [Char] -> [Char]
    In the expression: ('#' :)
    In the list element: ('#' :)

Writing the function as:

bla = words [ ('#':) ]

produces the correct error message with [Char] instead of `w_a1bp'.

I was doing this with GHC-6.4.2 on Ubuntu with a 2.6.17 kernel.