Export Snapshot

[Ray]

I may have found an intermittent bug in 6.5.1 using Windows 8.1; the 
following line of script works sometimes:

    export snapshot from rect myGlobalRect to myImageData as PNG

but other times it gives me this error:

    stack "My Stack": execution error at line 5 (export: no image 
selected, or image not open), char 1

The key thing here is that I am NOT exporting from an image or any other 
control, simply a rect in global coordinates from the screen, which in 
my case is still always over a Livecode stack.

I can't quite pinpoint how to repeat this either.  Seems to come and go 
randomly.  Ideas?