Pull request was open, original branch rebased and merged; now cannot close pull request

I have a pull request that I cannot close: https://bitbucket.org/metabrainz/musicbrainz-server/pull-request/350/mbs-6606-deny-edit-relationship-edits-when/diff

I think I originally created this branch, did my work and opened a pull request. Later, this branch was rebased and merged manually. As such, the original pull request was open. I force pushed the rebased branch, which didn't close the pull request because the pull request needs to be updated due to the rebase. However, when I go to the 'update' page I cannot update the pull request because there are no changes (because I manually merged this work to the target branch).