Failed auto-stacking on first push from 2a branch with non-2a trunk

$ bzr push lp:~gagern/trac-bzr/quoting
Using default stacking branch /~trac-bzr-team/trac-bzr/trunk at lp-46125520:///~gagern/trac-bzr
bzr: ERROR: KnitPackRepository('lp-46125520:///~trac-bzr-team/trac-bzr/trunk/.bzr/repository')
is not compatible with
CHKInventoryRepository('lp-46125520:///~gagern/trac-bzr/quoting/.bzr/repository')
different serializers

$ bzr push lp:~gagern/trac-bzr/quoting
This transport does not update the working tree of: bzr+ssh://bazaar.launchpad.net/~gagern/trac-bzr/quoting/. See 'bzr help working-trees' for more information.
Created new branch.

$ bzr info ..
Shared repository with trees (format: 2a)

So I'm pushing from a 2a branch to launchpad, to a project whose trunk seems to be in some older format, so the stacking fails. It would be nice if it wouldn't, without me jumping through hoops in order to match repo versions at both ends. And if stacking on an older format repository is absolutely completely impossible, then an automatic fallback to an unstacked branch would be preferable to this cryptic error message. At the very least, give the user a hint as to how to disable stacking.

On my second attempt, when I retry, it seems that some dirs left over from my first run cause bzr to not attempt auto-stacking, so that one succeeds, although the branch still isn't stacked. That kind of behaviour is somewhat confusing, and you might consider doing something about it, although it served me quite well in this instance.