AppleEvent handler failed

I have the following AppleScript to copy files to client systems using Remote Desktop:

tell application "Remote Desktop"
set targets to computer list "Testing"
set destination to "/Users/administrator"
set source to "/Users/allen/src/mutt-1.5.20.tar.gz"
set copyTask to make new copy items task with properties {location:specific folder, destination path:destination, copy items:source, bandwidth limit:0, conflict resolution:replace, should open:false, ownership:destination folder owner}
execute copyTask on targets
end tell

When I run it I get "Remote Desktop got an error: AppleEvent handler failed." number -10000 on "execute copyTask on targets". I can substitute the copy items task for other Remote Desktop tasks and they execute fine. I can copy the items with the Remote Desktop program. Sample scripts I've found use the same syntax that I have. I'm not an AppleScript expert so I don't know what the problem could be. Any suggestions?

Admin OS: 10.6.1 (also tried from 10.5.8 server)
Client OS: 10.5.8
ARD: 3.3.1 on admin and clients