HUGS error: Unresolved overloading

Can anyone shed some light on the following error? Thanks in advance. isSorted :: Ord a => [a] -> Bool isSorted [] = True isSorted [x] = True isSorted (x1:x2:xs) | x1 <= x2 = isSorted (x2:xs) | otherwise = False -------- Hugs session for: /usr/local/share/hugs/lib/Prelude.hs haskell/sort.hs Main> isSorted [1] True Main> isSorted [1,2] True Main> isSorted [2,1] False Main> isSorted [] ERROR: Unresolved overloading *** Type : Ord a => Bool *** Expression : isSorted [] --------