Locate the section for your github remote in the .git/config
file. It looks like this:
[remote "upstream"]
url = https://github.com/chrismckelt/webminder.git
fetch = +refs/heads/*:refs/remotes/upstream/*
Now add the line fetch = +refs/pull/*/head:refs/remotes/$$$$$$$$/pr/*
to this section. Obviously, change the github url to match your project's URL. It ends up looking like this:
$$$$ = origin or upstream