Listhandler caused error in "comment.module on line 996" ?

If there were more than one message in the mailing list with the same title, say:

(1) How Are You (original messgae)
(2) FW: How Are You (reply to first message)

I have Listhandler stripped the FW:, so messgae (2) will actually be the same as Message (1) in title.
After removing FW:, message (2) will become "How Are You"

and I use "Retrieve Now" in mailhandler to retrieve the mail, message (1) will be posted successfully into the Forum, but message (2) will not. (Even though mailhandler said 2 messages were retrieved.

And if I check the log (watchdog), I will see the following errors (2 of them):
------
Type php
Location /drupal/admin/mailhandler/retrieve/1
Message Missing argument 2 for comment_save() in /home/website/public_html/drupal/modules/comment.module on line 996.
Severity error
-------
Type content
Location /drupal/admin/mailhandler/retrieve/1
Message Comment: modified .
Severity notice

I have verified this "bug?" by purposely adding extra text like this:
Message --> FW: How Are You2
and this new message will be posted fine.
(I have tried listhandler 4.6 and the cvs version, and get the same problem.)

Thanks.