Index: wp-includes/default-widgets.php
===================================================================
--- wp-includes/default-widgets.php	(revision 17231)
+++ wp-includes/default-widgets.php	(working copy)
@@ -747,7 +747,7 @@
 	}
 
 	function update($new_instance, $old_instance) {
-		$testurl = ( isset($new_instance['url']) && ($new_instance['url'] != $old_instance['url']) );
+		$testurl = ( isset($new_instance['url']) && ( ! isset($old_instance['url']) || ($new_instance['url'] != $old_instance['url']) ) );
 		return wp_widget_rss_process( $new_instance, $testurl );
 	}