Accessing properties through 'root'

Within a class that represents ‘slide.panel’, and is added to the display list, I have code that tries to access a public variable I set in the document class, like so:

root.satellites.push(sat);

‘satellites’ is the public variable, of course.

Flash throws the error; 1119: Access of possibly undefined property satellites through a reference with static type flash.display:DisplayObject.

I got around this problem by making the public variables static and using ‘Main.satellites’ (Main being the document class) instead of ‘root.satellites’ - but I would still like to know what I am doing wrong, if anything? Thanks for any help.