Incoming Webhook verification status 500

Crashlytics (or fabric.io) offers an integration with Slack or custom webhooks.
Mattermost webhooks are described as compatible to Slack, but unfortunately it seems that not any service with Slack integration is compatible to mattermost.

Fabric uses an verification webhook. They send some request to the webhook including following payload:

{"event": "verification", "payload_type": "none"}

Using curl:

 curl -i -X POST -d 'payload={"event": "verification", "payload_ty": "none"}' https://example.com/hooks/abc

Mattermost responses with status code 500:

HTTP/1.1 500 Internal Server Error
[...]
X-Ratelimit-Limit: 10
X-Ratelimit-Remaining: 9
X-Ratelimit-Reset: 1
X-Request-Id: s64pjxn8gjb35xeghxyd849q4h
X-Version-Id: 2.0.0.1456450262

and following response body:

{"id":"web.incoming_webhook.text.app_error","message":"No text specified","detailed_error":"","request_id":"s64pjxn8gjb35xeghxyd849q4h","status_code":500,"is_oauth":false}

More infomation: http://support.crashlytics.com/knowledgebase/articles/102391-how-do-i-configure-a-custom-web-hook

As result Fabric/Crashlytics can not be integrated with Mattermost

I do not known if other services rely on such an verification event to verify the access of an webhook.