It appears that I introduced a slight behavior change in d6bbb19, where previously a custom http.Agent instance would not be required to have its own protocol field, now it is mandatory.

This ends up being a problem for custom-defined http.Agent instances that don't necessarily subclass from the core http.Agent class, like my agent-base module.

First reported in TooTallNate/node-proxy-agent#1. I'll have a patch ready shortly.