[Haskell-beginners] Non-exhaustive patterns

Hey guys, Thiis is my function below: notThe :: String -> Maybe String notThe word       | word /= "the" = Just word       | word == [] = Just []       | otherwise = Nothing replaceThe :: String -> String replaceThe word = go (words word)       where go (x:xs)               | notThe x == Just [] = []               | notThe x == Just word = word ++ go xs               | notThe word == Nothing = " a " ++ go xs

replaceThe "what" "what*** Exception: chap12.hs:(13,13)-(16,55): Non-exhaustive patterns in function go

I thought I covered all the potential patterns in my replaceThe function. I'm not sure what pattern I've missed, any thoughts? Best regards, Jim