clamav: ERROR : Not supported data format

[vu pham]

I just installed MailScanner with SpamAssasin and ClamAV. For each
incoming email, /var/log/maillog shows 

Jun 24 22:11:29 home2 MailScanner[3471]: New Batch: Scanning 1 messages,
545 bytes 
Jun 24 22:11:29 home2 MailScanner[3471]: Virus and Content Scanning:
Starting 
Jun 24 22:11:32 home2 MailScanner[3471]: ERROR: Not supported data
format 
Jun 24 22:11:32 home2 MailScanner[3471]: Uninfected: Delivered 1
messages 

Googling shows it is one of the errors returning by ClamAV but could not
find out the solution.

Anybody can share your experience ?

Thanks,

Vu