diff --git a/pyads/symbol.py b/pyads/symbol.py
index 18590c5..76da02b 100644
--- a/pyads/symbol.py
+++ b/pyads/symbol.py
@@ -252,10 +252,10 @@ def add_device_notification(
         """
 
         if attr is None:
-            if self.plc_type is not None:
-                attr = NotificationAttrib(length=sizeof(self.plc_type))
-            elif self.structure_def is not None:
+            if self.structure_def is not None:
                 attr = NotificationAttrib(length=self._structure_size)
+            else:
+                attr = NotificationAttrib(length=sizeof(self.plc_type))
 
         handles = self._plc.add_device_notification(
             (self.index_group, self.index_offset), attr, callback, user_handle