Problem integrating ClamAV into Amavis

I'm running an rather old version of Amavis (not able to upgrade it at
this time). I've been using amavis with uvscan, but am now trying to
integrate clamav to replace uvscan.

I've been very careful to verify that the amavis stanza in
amavisd.conf points to the same socket file as clamd.conf specifies,
I've configured clamav to run as user amavis, and I've set ownership
of the clamav files to amavis.

When I start amavis I get the following in amavis.log:
Jul 10 10:02:16 pd.iowatelecom.net amavisd[7280]: (07280-01) WARN: all
primary virus scanners failed, considering backups

The appropriate section of amavisd.conf looks like:
['Clam Antivirus-clamd',
\&ask_daemon, ["CONTSCAN {}\n", "/var/run/clamav/clamd.ctl"],
qr/\bOK$/, qr/\bFOUND$/,
qr/^.*?: (?!Infected Archive)(.*) FOUND$/ ],

clamd.conf contains:
LocalSocket /var/run/clamav/clamd.ctl

so you can see that both clamav and amavis are looking at the same
socket. Any clues as to why amavis isn't recognizing the existence of
clamd running on the specified socket?