First merge-upstream command try to remove debian/ directory in previously merging branch

Here are the steps to reproduce:

mkdir branch-trunk
cd branch-trunk/
bzr init .
echo "foo" > foo
bzr add
bzr commit -m "0.1 release"
bzr tag 0.1
cd ..
bzr branch branch-trunk/ branch-packaging
cd branch-packaging/
dh_make -p "branch_0.1" --createorig
bzr add
bzr commit -m "first packaging"
bzr tag upstream-0.1
cd ../branch-trunk/
echo "bar" > foo
bzr commit -m "bar is better"
bzr tag 0.2
cd ..
tar -czf branch_0.2.orig.tar.gz branch-trunk/
cd branch-packaging/
bzr merge-upstream ../branch_0.2.orig.tar.gz ../branch-trunk/ --version 0.2
-> No distribution specified, and no changelog, assuming 'debian'
bzr: ERROR: You did not specify --package, and there is no changelog from which to determine the package name, which is needed to know the name to give the .orig.tar.gz. Please specify --package.

Indeed, as bzr st show that debian/ directory has been removed.