Here are some bugs that appear related (but I don't know whether they're strict dups, so not marking them as such):
* Bug 242175: Better error message when merging into empty branch [requests a (better) error message, absent a real fix]
* Bug 522740: Merging a branch into empty branch shows negative revision numbers in bzr log [a different symptom]
* Bug 297768: switch does't work with empty branches
* Bug 670291: Branching the 0. revision leads to weird behaviour ["switch" again]
Here are some bugs that appear related (but I don't know whether they're strict dups, so not marking them as such):
* Bug 242175: Better error message when merging into empty branch [requests a (better) error message, absent a real fix]
* Bug 522740: Merging a branch into empty branch shows negative revision numbers in bzr log [a different symptom]
* Bug 297768: switch does't work with empty branches
* Bug 670291: Branching the 0. revision leads to weird behaviour ["switch" again]