I have wasted like 2.5 days on this and I'm not sure where this error is caused.  I'm betting my money that some of the test library can't really handle binary file stream or something strange.  API call is getting executed and end graciously as far as I could tell but inner anonymous function doesn't get called in hear due to that strange error so it is somewhere between receiving API call back from '/diff/image' and calling anon function call back.