Export snapshot to an image?

[Graham Samuel]

I've been extracting bits of images using 'export snapshot', and this goes fine if the export is to a file. However, I have now tried to do an export to an image and nothing seems to happen. The command has the form

export snapshot from rect a,b,c,d of img "LargeBitmap" of cd "BitMapStore" of stack "FullSizeBitMap" to img "TestImage" of cd 1 of stack "Test"

All the objects and quantities in this script fragment have been tested as being of correct value and existence, and indeed the same 'from' values can export a valid file.

The docs seem to say that what I'm trying to do is legit - what am I doing wrong?

TIA

Graham

PS If anyone knows any other way of non-destructively copying a part of an image without munging the clipboard, I'd be more than happy to use it.