You have found a bug! It has been fixed for a while, but I haven't released an updated version yet. The two patches to fix it are:
http://neil.brown.name/git?p=mdadm;a=commitdiff;h=2ed4f75388f99968be58097941a9704f6e42d701
and
http://neil.brown.name/git?p=mdadm;a=commitdiff;h=f98841b3852ceb7fce56a6f818236a4af9b5a00a
If you have 'git' installed, you might do best to:
git clone git://neil.brown.name/mdadm mdadm cd mdadm make install
and try that.
