2006 CTS Nav DVD won't play

Hi guys,

I just purchased my first Cadillac, a 2006 CTS with all the trimmings.

I've been fiddling and I decided to try the DVD and was dissapointed that it simply doesn't work. I have the Bose sound system and the nav unit to go with it and when I insert a normal, retail (movie) DVD into the nav/dvd slot I get the message

"This type of disc can not be played"

If I insert an alternate region DVD, I get the correct "Region not correct" message so it is reading correctly.

Also, the nav dvd and nav function correctly.

Am I doing something stupid, or is something up with the unit?

Thanks guys!