lib/PublicInbox/WwwHighlight.pm | 1 +

diff --git a/lib/PublicInbox/WwwHighlight.pm b/lib/PublicInbox/WwwHighlight.pm
index 3d6ca03bdf5c3bc6e82ea134708779e2fe0fc6b8..09fc48aba0c1bce2f43a26e51c460dbe91483bb0 100644
--- a/lib/PublicInbox/WwwHighlight.pm
+++ b/lib/PublicInbox/WwwHighlight.pm
@@ -52,6 +52,7 @@ 		return \$buf if $r == 0;
 		$off += $r;
 	}
 	$env->{'psgi.errors'}->print("input read error: $!\n");
+	undef;
 }
 
 # entry point for PSGI