skip messagecache if message_id is not set

I just got this strange error:

Warning: fopen(temp/d035681a53baea32579f544f677849f6/) [function.fopen.html]: failed to open stream: Is a directory in /home/web/spider007.net/webmail/roundcubemail/program/steps/mail/func.inc on line 1207 PHP Error in [...]roundcubemail/program/steps/mail/func.inc (1214): Failed to write to temp dir

It seems that, for some strange reason on some strange messages, $message_id is not set, therefore this line in roundcubemail/program/steps/mail/func.inc fails:

$cache_path = $cache_dir.'/'.$message_id;

might it be a good idea to return false if message_id is not set, it seems that to me that the caching is not vital for the functioning of Roundcube?

Regards, Sjon