Can't select destination branch in pull request to upstream repository (BB-8757)

I have a fork of a git repository I need to submit some changes to.

I when I create the Pull Request on the Bitbucket WebUI, I can select the source branch (username/forkrepo/events-squash), but I can't enumerate the destination branch I want to merge it with (username/upstreamrepo/events).

The drop-down for selecting forkrepo branches is populated, but Bitbucket doesn't enumerate the branches for upstreamrepo.

Please see the attached screenshots. Problem occurs on both Chrome 27.0.1453.110 m and Firefox 6.0