How do I enable submodule cloning in git-buildpackage

We've been using `git-buildpackage` to build Debian packages with great success, but we've recently hit a wall since introducing git submodules into the mix. We normally run ``` git-buildpackage -b -us -uc --git-export-dir="latest" ``` to build the package. However, the contents of any submodules are not checked out and included in the package (we just get an empty directory where the submodule would be, just as we would if we cloned without the `--recursive` flag). It appears from the `git-buildpackage` log that support for submodules was added in v0.5.0.20, but in version 0.5.22 there is a changelog entry saying "Disable submodule processing by default." This implies that there is a flag or switch we can set to enable it. But no amount of man-page and internet trawling has revealed it. I imagine submodules are very common in Debian source code - but for the life of me I can't figure out how get the packages to build properly. Is my problem caused by the fact I'm telling it to do a binary-only package (the `-b` switch)?