diff --git a/objects/screen.c b/objects/screen.c
index 5b701f29..f0943c78 100644
--- a/objects/screen.c
+++ b/objects/screen.c
@@ -1222,6 +1222,9 @@ luaA_screen_swap(lua_State *L)
             if(ref_s && ref_swap)
                 break;
         }
+        if(!ref_s || !ref_swap)
+            return luaL_error(L, "Invalid call to screen:swap()");
+
         /* swap ! */
         *ref_s = swap;
         *ref_swap = s;