diff --git a/tensorflow_federated/python/aggregators/mean_factory.py b/tensorflow_federated/python/aggregators/mean_factory.py
index 2d75197ee5a6a14a499d50a472d388722a4fa7e9..ac85bc28d415993e4d71a83c0bd0a51c051cea76 100644
--- a/tensorflow_federated/python/aggregators/mean_factory.py
+++ b/tensorflow_federated/python/aggregators/mean_factory.py
@@ -58,7 +58,7 @@ class MeanFactory(factory.AggregationProcessFactory):
       self,
       value_sum_factory: Optional[factory.AggregationProcessFactory] = None,
       weight_sum_factory: Optional[factory.AggregationProcessFactory] = None,
-      no_nan_division: Optional[bool] = False):
+      no_nan_division: bool = False):
     """Initializes `MeanFactory`.
 
     Args: