git describe is failing

Hi,

I have a test repo which I'm playing with. It has about 15 commits and
one branch only (master) and couple of tags. Somehow it got to a state
when

$ git describe

gives:

fatal: cannot describe 'SHA-1'

The command is working ok with my other repo. It doesn't seem that the
test repo is corrupted. I can commit, push and pull, see the history
with gitk... How can I check or repair it?

Thanks,
Eugene