Index: wp-includes/widgets.php
===================================================================
--- wp-includes/widgets.php	(revision 22133)
+++ wp-includes/widgets.php	(working copy)
@@ -845,10 +845,12 @@
 		$index = "sidebar-$index";
 	} else {
 		$index = sanitize_title($index);
-		foreach ( (array) $wp_registered_sidebars as $key => $value ) {
-			if ( sanitize_title($value['name']) == $index ) {
-				$index = $key;
-				break;
+		if ( empty( $wp_registered_sidebars[$index] ) ) {
+			foreach ( (array) $wp_registered_sidebars as $key => $value ) {
+				if ( sanitize_title($value['name']) == $index ) {
+					$index = $key;
+					break;
+				}
 			}
 		}
 	}