diff --git a/telegram/base.py b/telegram/base.py
index f86f1433a..db7c231a7 100644
--- a/telegram/base.py
+++ b/telegram/base.py
@@ -64,7 +64,8 @@ class TelegramObject(object):
         """
         data = dict()
 
-        for key, value in self.__dict__.items():
+        for key in iter(self.__dict__):
+            value = self.__dict__[key]
             if value or value == '':
                 if hasattr(value, 'to_dict'):
                     data[key] = value.to_dict()