moving rectangles in WPF, Errors.

I am attempting to move rectangles in WPF inside the program while the program is running

I have:

leftrec.SetValue(Canvas.TopProperty, lefthand.Y);

Throws an "Argumentexception was unhandled" error, saying that "893 (among other values it's given me over the course of several debuggings) is not a valid value of property "Top"

my canvas is 1280 x 960....

Im stumped as to how to fix this.