Error pushing to bitbucket (changegroup.csetlog hook)

Every push for about the last week or so we are seeing this error printed to the console:

{{{
$ hg push
pushing to https://bitbucket.org/myuser/myrepo/\\
searching for changes
remote: adding changesets
remote: adding manifests
remote: adding file changes
remote: added 1 changesets with 2 changes to 2 files
remote: error: changegroup.csetlog hook raised an exception: 'NoneType' object has no attribute '_repo'
}}}

We have e-mail service integration on our private repo, if this helps pinpoint the problem.

The push appears to succeed, but no e-mail is generated.