Sending dwg file to a friend?

A friend and I both have Carlson software. He sent me a survey drawing by email to review and I'm unable to open the dwg file. The error message reads "Drawing file is not valid".
Carlson Software Discussion Group
http://update3.carlsonsw.com/forum/
http://update3.carlsonsw.com/forum/viewtopic.php?f=20&t=3367