Illustrator Scripting :: Script Suddenly Halts After Setting Crop Box

I'm working on a custom batch exporter, and at one point I need to set the crop box (inside an iteration of the current user-selection). Here's the relevant code:
 
var maxRect = getMaxRect ( objRef, overlappingItems );
alert ( maxRect.left.toString ( ) + " " + maxRect.top.toString ( ) + " " + maxRect.width.toString ( ) + " " + maxRect.height.toString ( ) );
doc.cropBox = [ maxRect.left, maxRect.top, maxRect.left + maxRect.width, maxRect.top + maxRect.height ];
alert ( "Crop box set" );
 
getMaxRect is a function that calculates the maximum bounding box between the current object ref and a group of overlapping items that I've already computed. The alert on the second line gives a nice set of numbers, so I'm fairly confident that the code thus far is ok. (In addition, I've inspected the crop marks subsequent to the execution of the above lines, and it looks all right.)
 
Here's the thing. Once doc.cropBox (doc being the active document) is set, it seems like the script execution simply halts. The crop box is actually set in the active document, but it seems to stop after setting this. I never see the second alert, and I don't see any error messages either.