Well, this Machine Image ( projects/debian-cloud/global/images/debian-10-buster-v20210721 ) was the default value set by the WebUI (not something we specified). A ccording to the docs it should be an optional field? However, when I clear the field and try to re-launch I get this error:

googleapiclient.errors.HttpError: <HttpError 400 when requesting 

 returned "Invalid value for field 'resource.disks[0].initializeParams.sourceImage': ''. The URL is malformed.". Details: "[{'message': "Invalid value for field 'resource.disks[0].initializeParams.sourceImage': ''. The URL is malformed.", 'domain': 'global', 'reason': 'invalid'}]">