diff --git a/mongoengine/document.py b/mongoengine/document.py
index 01b69fb2..7b05ef8c 100644
--- a/mongoengine/document.py
+++ b/mongoengine/document.py
@@ -152,7 +152,7 @@ class Document(BaseDocument):
         """
 
         def fget(self):
-            if not 'id_field' in self._meta:
+            if 'id_field' not in self._meta:
                 return None
             return getattr(self, self._meta['id_field'])