LearnBoost/engine.io-client — pull request #132 by asdfryan (2013-03-05 20:10:51+00:00)
Title: Issue 83 fix: IE9/10 error

The problem was that this.xhr didn't exist. OP of the issue suspects that cleanup is getting called twice (once by onSuccess, the other by onError). I think just returning if the xhr is undefined/null would fix this (since there's nothing to cleanup).