Invalid Payload JSON” When User Sends a URL (Instagram → Webhook)

I’ve run into an issue with my Instagram + ManyChat automation that sends user messages to an external webhook.

Everything works perfectly when the user sends normal text messages,  but when the message contains a URL, the automation fails and logs this error:

Level: error  
Description: Invalid payload json  
Type: External Request Processing Events  

Here’s the payload I’m sending to my webhook:

{
  "userId": "{{user_id}}",
  "first_name": "{{first_name}}",
  "message": "{{last_input_text}}"
}

And the error only appears when {{last_input_text}} includes a link (like https://example.com).