bad header

Using: RoundCube-20051021 postfix-2.2.2-2 cyrus-imapd-2.2.12-6.fc4 amavisd-new-2.3.2-1.2.fc4.rf apache-2.0.54-10.2 Linux Fedora Core 4

I configured Postfix to use amavisd-new as a scanning hub (using SpamAssassin for spam and ClamAV for viruses). When composing messages in RoundCube and sending them through Postfix/Amavisd-new, the messages get flagged by amavis as "bad header":

X-Amavis-Alert: BAD HEADER Improper use of control character (char 0D hex) in message header 'MIME-Version': MIME-Version: 1.0\r\n

What's the cause?

Do you need any other info to diagnose this issue?