osc branch of revision results in error

Hi, when I branch an earlier revision of a package like osc branch -r 36 -f devel:languages:python python-cairocffi <target> I get the error: BuildService API error: failed to branch: conflict in file python- cairocffi.spec Well, it is expected that the spec file of the reveision deviates from the actual package. So how can I branch an earlier version? Cheers Axel