diff --git a/src/ai.c b/src/ai.c
index 1cc0a2c..3ec9f23 100644
--- a/src/ai.c
+++ b/src/ai.c
@@ -1028,6 +1028,7 @@ static int ai_face(lua_State* L) {
   }
   else if(lua_isvector(L, 1))
     lv = lua_tovector(L, 1);
+  else LLUA_INVALID_PARAMETER();
 
   mod = 10;