diff --git a/src/assets.c b/src/assets.c
index f9502d5..9740593 100644
--- a/src/assets.c
+++ b/src/assets.c
@@ -135,7 +135,7 @@ asset_id get_asset_id(void* asset) {
         }
     }
 
-    RETURN_WARNING(NULL, "Failed to find asset referencing %p", asset);
+    RETURN_WARNING(0, "Failed to find asset referencing %p", asset);
 }
 
 void free_asset(asset_id id) {