Index: wp-includes/widgets.php
===================================================================
--- wp-includes/widgets.php	(revision 24863)
+++ wp-includes/widgets.php	(working copy)
@@ -577,6 +577,23 @@
 }
 
 /**
+ * Checks if a sidebar is registered.
+ *
+ * @since  3.7.0
+ *
+ * @param string $name The ID of the sidebar when it was added.
+ *
+ * @return boolean True if the sidebar is registered, false otherwise.
+ */
+function is_registered_sidebar( $name ) {
+	global $wp_registered_sidebars;
+
+	if ( isset( $wp_registered_sidebars[$name] ) )
+		return true;
+	return false;
+}
+
+/**
  * Removes a sidebar from the list.
  *
  * @since 2.2.0
@@ -588,7 +605,7 @@
 function unregister_sidebar( $name ) {
 	global $wp_registered_sidebars;
 
-	if ( isset( $wp_registered_sidebars[$name] ) )
+	if ( is_registered_sidebar( $name ) )
 		unset( $wp_registered_sidebars[$name] );
 }