Error: cb is not a function

[Blagus]

When working on TFTP provisioning with an IP phone, combined with HTTP proxy from examples, I started getting following error: "Error: cb is not a function".
It was being sent from "error" event ( req.on("error"...); ).
The error was exclusive to the phone, transfer to another PC/server worked fine.

I solved the error by changing lib/protocol/writer.js line 251 from
this._cb = null;
to
this._cb = function() { return false; };

This seems to work fine in my case, but is there a better solution?
How do I find out what's actually causing this error to show up only when sending files to the phone?