vc in emacs problem with git

This is emacs 23 using the new 'updated-for-modern-dvcs' vc
I see a mode line saying (note the colon)
Git:master

In the info on vc (Version control and the mode line) the emacs manual says:

  The character between the back-end name and the revision ID
indicates the version control status of the file.  `-' means that the
work file is not locked (if locking is in use), or not modified (if
locking is not in use).  `:' indicates that the file is locked, or that
it is modified.

However at the shell a git status says
# On branch master
nothing to commit (working directory clean)

If I close and reopen the file the colon becomes a -
But I get a message:

`working-revision' not found: using the old `workfile-version' instead.