diff --git a/graphene/types/datetime.py b/graphene/types/datetime.py
index 205ae7a9..d2dd27bf 100644
--- a/graphene/types/datetime.py
+++ b/graphene/types/datetime.py
@@ -14,7 +14,7 @@ class DateTime(Scalar):
 
     @staticmethod
     def serialize(dt):
-        assert isinstance(dt, datetime.datetime)
+        assert isinstance(dt, (datetime.datetime, datetime.date)), 'Received not compatible datetime "{}"'.format(repr(dt))
         return dt.isoformat()
 
     @staticmethod