diff --git a/gcloud/datastore/query.py b/gcloud/datastore/query.py
index d15e27b78468..7939028dac75 100644
--- a/gcloud/datastore/query.py
+++ b/gcloud/datastore/query.py
@@ -207,7 +207,8 @@ def add_filter(self, property_name, operator, value):
 
         :type value: :class:`int`, :class:`str`, :class:`bool`,
                      :class:`float`, :class:`NoneType`,
-                     :class:`datetime.datetime`
+                     :class:`datetime.datetime`,
+                     :class:`gcloud.datastore.key.Key`
         :param value: The value to filter on.
 
         :raises: :class:`ValueError` if ``operation`` is not one of the