Branch Merger Trigger Webhook with no Branch Info

I use BitBucket with DeployHQ and Flowdock.
When we push to BitBucket, it notifies both service for a build and a notification.

I noticed that if I merge a branch together on SourceTree and push it back to Bitbucket, BitBucket will fire off events to webhooks with branch name as "null".

You can see from the screenshot of Flowdock. If it's a new commit, it is fine (dev branch in screenshot) but when I merge the changes into other branch and push, Flowdock will report as null...

DeployHQ also uses the branch info to trigger build for different branches... so it doesn't know which branch to build when I merge.