Excel script to save as CSV

I have been pounding my head over this one for a couple of weeks. I've 
been playing with it a few minutes a day. I am simply trying to drop an 
..xls file on my script and have it save the dropped script in the same 
directory as the original as a .csv file. Sounds simple right? Well 
this script works the first time around, sort of. It will do exactly 
that, except the resulting .csv file is empty. If I run it again, it 
tells me that 'active workbook' doesn't understand the 'save' command. 
I don't understand… Can someone help?

Here is the script:

on open fileList
	repeat with aFile in fileList
		set aFile to (aFile)
		tell application "Microsoft Excel"
			open aFile
			save ActiveWorkbook in (aFile & ".csv") as CSV
		end tell
	end repeat
end open


Thanks in advance.