Folder sizes in Snow Leopard

I’ve got a couple of scripts that are broken in Snow Leopard because of what I think might be a change to the properties that Finder returns for folders.

Take this super-simple script.

tell application "Finder"
	set theItemAlias to choose folder
	set theSize to size of theItemAlias
	repeat while (theSize is missing value)
		set theSize to size of theItemAlias
	end repeat
end tell

In Leopard, the size property was sometimes returned as ‘missing value’, hence the loop. In Snow Leopard, I can leave the loop running indefinitely but it won’t return the size.

Am I doing something wrong here or has something changed?

Additional information: I realise that I can use the following script, but ‘info for’ is slower than accessing properties directly so I’d rather avoid it if I can!

tell application "Finder"
   set theItemAlias to choose folder
   set theSize to size of (info for theItemAlias)
end tell