~/www> git branch --track live origin/master
fatal: Not a valid object name: \'origin/master\'.
~/www> git remote
origin
~/www> git branch
* master
test_branc
$ git branch -r
origin/1.x
origin/1.x@60
origin/1.x@63
origin/HEAD -> origin/master
origin/master
$ git branch --track live origin/blah
fatal: Not a valid object name: 'origin/blah'.
As has been suggested you can only track a remote if it has been added. Perhaps add the remote like this
$ git remote add upstream git://github.com/svnpenn/rtmpdump.git
$ git fetch upstream
Example