Css validation

I get the following error when I validate a page "including" the  CSS>

Please, validate your XML document first!

Line 32

Column 5

The string "--" is not permitted within comments.



If I validate the XHTML document alone it passes and if I validate the 
CSS alone it passes. I have a "<!--#include file="menu.txt"-->" entry 
that passes on the HTML validator but ALWAYS fails when combined with 
the CSS from a live URL. I cannot find any instance of "--" in either 
the page or the menu.txt file.

What do I need to do to get this to validate?

Thank you for your time and consideration,


Sally K