I verified that:
Further investigation shows:
response.status : 0
The code for this.success() used by respondToReadyState() treats this as a response to call this.options.onSuccess, but I suspect it would be better handled by this.options.onFailure or by dropping it or something.
See https://stackoverflow.com/questions/36416389/why-does-my-xmlhttprequest-have-readystate-4-but-status-0 for reports of this case.