diff --git a/homeassistant/components/ads/__init__.py b/homeassistant/components/ads/__init__.py
index 060e9b2b987923..1b90e645af4dc1 100644
--- a/homeassistant/components/ads/__init__.py
+++ b/homeassistant/components/ads/__init__.py
@@ -171,13 +171,12 @@ def add_device_notification(self, name, plc_datatype, callback):
             hnotify, huser = self._client.add_device_notification(
                 name, attr, self._device_notification_callback)
             hnotify = int(hnotify)
+            self._notification_items[hnotify] = NotificationItem(
+                hnotify, huser, name, plc_datatype, callback)
 
         _LOGGER.debug(
             "Added device notification %d for variable %s", hnotify, name)
 
-        self._notification_items[hnotify] = NotificationItem(
-            hnotify, huser, name, plc_datatype, callback)
-
     def _device_notification_callback(self, notification, name):
         """Handle device notifications."""
         contents = notification.contents
@@ -187,9 +186,10 @@ def _device_notification_callback(self, notification, name):
         data = contents.data
 
         try:
-            notification_item = self._notification_items[hnotify]
+            with self._lock:
+                notification_item = self._notification_items[hnotify]
         except KeyError:
-            _LOGGER.debug("Unknown device notification handle: %d", hnotify)
+            _LOGGER.error("Unknown device notification handle: %d", hnotify)
             return
 
         # Parse data to desired datatype