--username wrong or ignored for git-svn

Hello,

Just now I attempted to commit some changes that a coworker had made
to our subversion repository using git-svn.  Since it was his work, we
attempted to commit it using his username, though it was done in a git
repository that I'd previously used with my username.  So I did:

git-svn dcommit --username <name>

But it ignored --username and the commit is now attributed to me.
Is it because I'd previously done dcommit in my own username?
There was no warning that it wouldn't do what I expected.

Admittedly it's was a weird thing to have him make a change in my
local repository but we didn't think it would be a problem if we
carefully used --username.

thanks,
Steve