I was getting that identical error also.
I followed the instruction in the google link about changing the clamassassin file i.e:
CLAMSCANOPT="--no-summary --stdout --mbox"
Change this to:
That appears to have fixed the issue. My email header now shows:
X-Virus-Checker-Version: clamassassin 1.2.3 with clamscan / ClamAV 0.90/2631/Thu Feb 22 16:33:11 2007