Index: src/wp-includes/class-wp-network.php
===================================================================
--- src/wp-includes/class-wp-network.php	(revision 37657)
+++ src/wp-includes/class-wp-network.php	(working copy)
@@ -28,8 +28,9 @@
 	 * A numeric string, for compatibility reasons.
 	 *
 	 * @since 4.4.0
+	 * @since 4.6.0 Type changed from string to int.
 	 * @access public
-	 * @var string
+	 * @var int
 	 */
 	public $id;
 
@@ -156,6 +157,8 @@
 				return $this->blog_id;
 			case 'site_id':
 				return (int) $this->blog_id;
+			case 'id';
+				return (int) $this->id;
 		}
 
 		return null;