[Haskell-beginners] merge error

The below produces an error. And I am very proud that I could use the GHCi debugging tools to get this far. merge [] [] works. merge [1] [] works. I don't know why the failing example fails. It should return: [4,5] Help to unstuck is appreciated. :step merge [4,5] [] *** Exception: ex6_8.hs:(12,1)-(16,66): Non-exhaustive patterns in function merge Given: merge :: Ord a => [a] -> [a] -> [a] merge [] [] = [] merge [x] [] = [x] merge [] [y] = [y] merge first@(x:xs) second@(y:ys) | x <= y = x : merge xs second | otherwise = y : merge first ys