diff --git a/flip-clock.html b/flip-clock.html
index 1ac6bef..52fe8ae 100644
--- a/flip-clock.html
+++ b/flip-clock.html
@@ -103,6 +103,12 @@
      * @default null
      */
     timer: null,
+    /***
+     * @private
+     * @type Boolean
+     * @default false
+     */
+    isRunning: false,
 
     properties: {
       /**