Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 18658)
+++ wp-includes/taxonomy.php	(working copy)
@@ -2115,6 +2115,7 @@
 
 	$tt_ids = array();
 	$term_ids = array();
+	$new_tt_ids = array();
 
 	foreach ( (array) $terms as $term) {
 		if ( !strlen(trim($term)) )
@@ -2137,9 +2138,11 @@
 		do_action( 'add_term_relationship', $object_id, $tt_id );
 		$wpdb->insert( $wpdb->term_relationships, array( 'object_id' => $object_id, 'term_taxonomy_id' => $tt_id ) );
 		do_action( 'added_term_relationship', $object_id, $tt_id );
+		$new_tt_ids[] = $tt_id;
 	}
+	if (count($new_tt_ids))
+		wp_update_term_count($new_tt_ids, $taxonomy);
 
-	wp_update_term_count($tt_ids, $taxonomy);
 
 	if ( ! $append ) {
 		$delete_terms = array_diff($old_tt_ids, $tt_ids);