Index: wp-includes/widgets.php
===================================================================
--- wp-includes/widgets.php	(revision 48922)
+++ wp-includes/widgets.php	(working copy)
@@ -1369,7 +1369,9 @@
 							}
 
 							// Make sure this sidebar wasn't mapped and removed previously.
-							if ( ! empty( $existing_sidebars_widgets[ $sidebar ] ) ) {
+                            // Make sure the sidebar also exists on the new sidebar,
+                            // Otherwise send it to inactive sidebar
+							if ( ! empty( $existing_sidebars_widgets[ $sidebar ] ) && isset( $new_sidebars_widgets[ $sidebar ] ) ) {
 
 								// We have a match that can be mapped!
 								$new_sidebars_widgets[ $new_sidebar ] = array_merge( $new_sidebars_widgets[ $new_sidebar ], $existing_sidebars_widgets[ $sidebar ] );