[Exim] sa-exim woes

I think I"ve almost got SA-Exim running now. But one thing is going
wrong still. No messages are being scanned. The header always says
X-SA-Exim-Scanned: No; SAEximRunCond expanded to false


but the only condition in /etc/exim4/sa-exim.conf is:
SaEximRunCond: 1


shouldn't that force SpamAssassin to Run?

What am I doing wrong?

--- Dan