MAV when unlocking workstation

I did a lock workstation under Windows XP and when I unlocked the machine, I got a MAV error.

This happened in an executable version of my program so I went back into the IDE to re-create it. The same thing happened. The Debugger pointed to a simple line command.

Next I started a new program with nothing more than a line command. The MAV occured again after unlocking. Has anyone else experienced this?

Graphics 640,480,32,2
SetBuffer BackBuffer()

While KeyHit(1) = 0 
   Cls
   Line 100,100,300,300  
   Flip False   
Wend
End

This is in version 1.96.