Index: wp-admin/widgets.php
===================================================================
--- wp-admin/widgets.php	(revision 10480)
+++ wp-admin/widgets.php	(working copy)
@@ -89,9 +89,10 @@
 	ob_end_clean();
 
 	// Prophylactic.  Take out empty ids.
-	foreach ( (array) $_POST['widget-id'] as $key => $val )
-		if ( !$val )
-			unset($_POST['widget-id'][$key]);
+	if ( isset($_POST['widget-id']) )
+		foreach ( (array) $_POST['widget-id'] as $key => $val )
+			if ( !$val )
+				unset($_POST['widget-id'][$key]);
 
 	// Reset the key numbering and store
 	$new_sidebar = isset( $_POST['widget-id'] ) && is_array( $_POST['widget-id'] ) ? array_values( $_POST['widget-id'] ) : array();