HUGS error: Unresolved overloading

[David Scarlett]

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

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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 []
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