[Haskell-beginners] Non-exhaustive patterns in function error

I have this intersect1 :: ([a],[a]) -> [a] intersect1 (s,[]) = [] interesct1 (s,t:ts) | elem t s = t : intersect1 (s,ts) | otherwise = intersect1 (s,ts) and when I try this intersect1 ([1,2],[1,2,3]) I get the error Non-exhaustive patterns in function intersect1 Not sure what's wrong with this. LB