diff --git a/src/Bergamot/Syntax.elm b/src/Bergamot/Syntax.elm
index a17f5c8..f09f8b4 100644
--- a/src/Bergamot/Syntax.elm
+++ b/src/Bergamot/Syntax.elm
@@ -65,7 +65,7 @@ unifyList l1 l2 us =
     case (l1, l2) of
         (t1 :: ts1, t2 :: ts2) ->
             unify t1 t2 us
-            |> Maybe.andThen (\(t, usp) -> unifyList ts1 ts2 usp)
+            |> Maybe.andThen (\(t, usp) -> Maybe.map (Tuple.mapFirst (\ts -> t :: ts)) (unifyList ts1 ts2 usp))
         ([], []) -> Just ([], us)
         _ -> Nothing