diff --git a/demo/time-picker-advanced-demos.html b/demo/time-picker-advanced-demos.html
index 2b80489..058f46b 100644
--- a/demo/time-picker-advanced-demos.html
+++ b/demo/time-picker-advanced-demos.html
@@ -70,6 +70,9 @@ <h3>Custom Validator</h3>
           window.addDemoReadyListener('#time-picker-advanced-demos-custom-validator', function(document) {
             class TimePickerWorkingHoursElement extends Vaadin.TimePickerElement {
               checkValidity() {
+                if (!this.value) {
+                  return false;
+                }
                 var hour = /^(\d+)/.exec(this.value)[1];
                 return hour >= 9 && hour < 17;
               }