custom remote commands fail

I've got drush 6.0-rc4 installed locally and on the server.

If I do "drush @example.prod -v custom-command 1375148040" where the 1375148040 is an argument to a command, then the job fails:

The drush command '1375148040' could not be found.  Run `drush           [error]
cache-clear drush` to clear the commandfile cache if you have
installed new extensions.

But I tried doing a "drush @example.prod -v cc drush" and that didn't help. And I tried sshing to the production webserver and from there when I do "drush @example.prod -v cc drush" or even the full "drush @example.prod -v custom-command 1375148040" - both of those work fine from the webserver.

Any ideas why this would fail after the upgrade from 5.4 to 6.0.0rc3 or 6.0.0rc4?