Index: node.module
===================================================================
--- node.module	(revision 19)
+++ node.module	(working copy)
@@ -1232,7 +1232,10 @@
       }
       if ($weight = (int)variable_get('node_rank_recent', 5)) {
         // Exponential decay with half-life of 6 months, starting at last indexed node
-        $ranking[] = '%d * POW(2, (GREATEST(MAX(n.created), MAX(n.changed), MAX(c.last_comment_timestamp)) - %d) * 6.43e-8)';
+        // c.last_comment_timestamp may be NULL. Since both MAX(anynumber, NULL) and
+        // GREATEST(anynumber, NULL) return NULL, we OR MAX(c.last_comment_timestamp) with 1
+        // to prevent it from being NULL.
+        $ranking[] = '%d * POW(2, (GREATEST(MAX(n.created), MAX(n.changed), MAX(c.last_comment_timestamp) || 1) - %d) * 6.43e-8)';
         $arguments2[] = $weight;
         $arguments2[] = (int)variable_get('node_cron_last', 0);
         $join2 .= ' LEFT JOIN {node_comment_statistics} c ON c.nid = i.sid';