`bzr ci --fixes blah-blah`: how it supposed to work?

[Alexander Belchenko]

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I starting to use trac for my bzr-controlled projects.
One thing that I don't understand, and documentation
don't tell me anything useful is the '--fixes' flag
of bzr commit command. And `bzr help bugs` is not
very useful, as well ;-)

I try to imagine why for it might be useful, but I don't
understand how it supposed to work?
For launchpad bugs the string lp:12345 is works,
as I remember.

But, what I do wrong in following session:

$ bzr ci --fixes=trac:15
bzr: ERROR: Unrecognized bug trac:15. Commit refused.

?

[µ]
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