Lars,
We have identified what the problem is, however, fixing this may a little more difficult than one would hope. The Outlook type that is used to display the field does not support empty fields. So it automatically populates it with '0'.
If we change the type to a string instead of an integer, it breaks sorting and other nice things to have.
Unfortunately, for now, this is something that is not fixed. Hopefully we can have this resolved in a future release.
Thanks,
~Joe