probot error on google cloud functions

hi i deployed my probot successfully on probot and was able to make call , but iam getting following response : {"statusCode":500,"body":"{\"message\":{\"code\":401,\"message\":\"{\\\"message\\\":\\\"Missing 'issuer' claim ('iss') in assertion\\\",\\\"documentation_url\\\":\\\"https://developer.github.com/v3\\\"}\"}}"}

any idea ?

The report names Probot running on Google Cloud Functions but does not identify files, tests, or deployment configuration. Start by reproducing the request and reviewing the GitHub authentication setup around the reported 401 response; done means the deployed function authenticates successfully without the missing issuer claim.

Written by the indexing model from the issue text.