Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 18388)
+++ wp-includes/taxonomy.php	(working copy)
@@ -1150,12 +1150,15 @@
 	global $wpdb;
 	$empty_array = array();
 
-	$single_taxonomy = false;
-	if ( !is_array($taxonomies) ) {
-		$single_taxonomy = true;
-		$taxonomies = array($taxonomies);
+	$single_taxonomy = true;
+	if ( is_array( $taxonomies ) && 1 < count( $taxonomies ) ) {
+		$single_taxonomy = false;
 	}
 
+	if ( is_string( $taxonomies ) ) {
+		$taxonomies = array( $taxonomies );
+	}
+
 	foreach ( $taxonomies as $taxonomy ) {
 		if ( ! taxonomy_exists($taxonomy) ) {
 			$error = & new WP_Error('invalid_taxonomy', __('Invalid Taxonomy'));