Simple script works when run, but not in Mail.app handler

Hi,

I am an Applescript beginner wannabe, but have done extensive digging
to try to answer my question, so please forgive me if everyone knows it
but me.

When I run this script,

tell application "Mail"
set path_to_file to "Macintosh HD:"
set path_to_file to path_to_file &
"Users:peterlawton:Desktop:MailFilterLog.txt"
set fileRef to open for access file path_to_file with write permission
close access fileRef
end tell

in Script Debugger, it runs fine as a standalone script. But when I
try it in Mail.app's perform_mail_action(info) handler, the "set
fileRef to open ..." fails with a -1712 error, "applescript timed out".
In case it matters, I am single-stepping in both cases.

Can anyone shed some light on this for me? Obviously, I don't care to
run this script as a standalone one.

TIA,

Peter Lawton