Fork sync issue, creates a merge commit with message that starts with "null"

The commit message looks weird when sync merges instead of fast forwards. Example from git log:

commit 140a801bb753019425fa87dd7576ef88028419a1
Merge: 779d6df 69d0ceb
Author: Kalle Kula <xxx@yyy.zzz>
Date: Wed Jul 24 14:15:38 2013 +0200

null

• commit '69d0ceb7a64e875c7f8c0dc0ec331a71c557c3f6':
  Added a line in mother repo

Workflow:

1. Make a fork (B) of repo A.
2. Make a commit in fork B and push it to Stash
3. Make another commit in repo A, but make sure it's NOT going to result in a merge conflict that the user needs to handle when synced to fork B, i.e change two different files.
4. Do a "manual" sync in Stash of the fork, since it couldn't be done automatically by Stash.
5. And here I get the "null" message in the fork.