Can't pass "challenge" while trying to connect webhook with Lambda

I receive a message from Monday : “URL don’t return solution”

I have Lambda function on Python:

def lambda_handler(event, context):
    
    body_str = event.get("body", "{}")
    body_str = body_str if body_str else "{}"
    body_obj = json.loads(body_str)
    a = body_obj.get("challenge", "")
    print({"challenge" :a})
    return {"challenge" :a}

My Cloud Logs show that what I return looks like that

Can you help me, maybe I returning it in a wrong way