Index: user.php
===================================================================
--- user.php	(revision 27299)
+++ user.php	(working copy)
@@ -176,7 +176,8 @@
 		$user_id = wp_update_user( $user );
 	} else {
 		$user_id = wp_insert_user( $user );
-		wp_new_user_notification( $user_id, isset( $_POST['send_password'] ) ? wp_unslash( $pass1 ) : '' );
+		if (!is_wp_error($user_id))
+			wp_new_user_notification( $user_id, isset( $_POST['send_password'] ) ? wp_unslash( $pass1 ) : '' );
 	}
 	return $user_id;
 }