diff --git a/objects/screen.c b/objects/screen.c
index d9a5993a..3321bd0d 100644
--- a/objects/screen.c
+++ b/objects/screen.c
@@ -458,7 +458,7 @@ luaA_screen_module_index(lua_State *L)
 {
     const char *name;
 
-    if((name = lua_tostring(L, 2)))
+    if(lua_type(L, 2) == LUA_TSTRING && (name = lua_tostring(L, 2)))
         foreach(screen, globalconf.screens)
             foreach(output, (*screen)->outputs)
                 if(A_STREQ(output->name, name))