Crop page with Acrobat?

I created a script to crop a page when it’s dropped into a watched folder and it goes as follows:

on adding folder items to this_folder after receiving added_items
	repeat with added_item in added_items
		tell application "Acrobat 5.0"
			open added_item
			set page bounds to {155, 155, 155, 155}
		end tell
	end repeat
end adding folder items to

The page bounds parts I set to 155 just as a test crop size. I’m getting an error each time I try to re-jig it. Any ideas why?