sa-learn error msg: isn't numeric in numeric lt

[Bart Lievers]

Hello,

I've searched the archives but i didn't find anything (maybe i'm blind)
But,

When I'm learing SA with sa-learn, sometimes i get this message:
Argument "\005337172gM-^PM-\r?" isn't numeric in numeric lt (<)
at /usr/lib/perl5/site_perl/5.8.0/Mail/SpamAssassin/BayesStore.pm line 1245.

What is it ? and what can i do about it.
When i debug, i find this line multiple times in my debug log.

I use
RH 8
MailScanner with SA 2.60

I've narrowed down which e-mail is doing this. But for the rest I haven't a
clou.