[CentOS] screen detatch

The man page for screen says that I can create a detatched screen running with a set command in it by doing this:

$ screen -dm $command

However, it doesn't work. Screen exits without creating the detached screen.

If I say

$ screen $command

...I get dropped into a screen session running $command as I would expect.

What's the magic invocation I'm missing?

Also, the next step will be for root to launch said screen session as someone else during boot time; am I asking for trouble by trying it?

# su - user -c screen -dmS $Label $command

Thanks for any insights or pointers to web resources I can use to learn from.