I find MC a bit pedantic sometimes. I think this happens when we merged a private version somewhere on someone disk with a trunk version then published. MC keep track of the merged private version and insist on having this branch when we further merge in Pharo. FYI, exactly the same happened to me when I tried to merge some Pharo version in squeak (an obscure DamienCassou branch was missing in pharo repository, can't remember which package though...). Instead of complaining instructively, MC was sending snapshot to nil... I just let it ignore the problem. In Pharo, you choosed to signal the problem with an explicit message, maybe that help fixing wrong configurations, but sometimes you should just have the option to ignore and proceed.
sure now I do not know what is the semantics of proceed. is it stop? Stef