diff --git a/backend/src/worker.rs b/backend/src/worker.rs
index 0b85262c92..4375ae8f1b 100644
--- a/backend/src/worker.rs
+++ b/backend/src/worker.rs
@@ -738,7 +738,7 @@ const args = await Deno.readTextFile("args.json")
 
 async function run() {{
     let res: any = await main(...args);
-    const res_json = JSON.stringify(res ?? null);
+    const res_json = JSON.stringify(res ?? null, (key, value) => typeof value === 'undefined' ? null : value);
     console.log();
     console.log("result:");
     console.log(res_json);