Sorry for re-opening this, but I'm having difficulty finding the bug in the bug tracker, and the newest version released today doesn't seem to include a fix for this. I'm wary of using the dev version because, well, it's a dev version.
The closest thing I can find in the bug tracker is issue #374, which is related but not quite the same. As per that issue, a value of 000000 is correctly allowed (and this does work for me), but a value of simply a single 0 is still not working for me.
I'm not able to set this field to a number, as the value can either be numeric or a descriptive string (such as "variable") depending on the specific item.
Have I missed the correct issue in the tracker?