git svn dcommit <branch> committed to trunk anyway

I expected git svn dcommit <branch> to push commits into the svn
branch called <branch> (a repo relative URL of
/branches/foobar1/foobar2/<branch>)

However, it decided to push into trunk instead, which was not the
intended effect.

What have I misunderstood about how git svn dcommit is supposed to work?

jon.