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