diff --git a/unity/test/Src/UnitTestLoader.cs b/unity/test/Src/UnitTestLoader.cs
index bc51dd96b4..8d73d1db40 100644
--- a/unity/test/Src/UnitTestLoader.cs
+++ b/unity/test/Src/UnitTestLoader.cs
@@ -13,6 +13,7 @@ public UnitTestLoader2()
 
         }
 
+        [UnityEngine.Scripting.Preserve]
         public bool IsESM(string filepath)
         {
             return !filepath.EndsWith(".cjs");