diff --git a/google/cloud/bigtable/row_set.py b/google/cloud/bigtable/row_set.py
index 0269d8761..32a9bd1e3 100644
--- a/google/cloud/bigtable/row_set.py
+++ b/google/cloud/bigtable/row_set.py
@@ -137,7 +137,7 @@ def _update_message_request(self, message):
         :param message: The ``ReadRowsRequest`` protobuf
         """
         for each in self.row_keys:
-            message.rows.row_keys.append(_to_bytes(each))
+            message.rows.row_keys._pb.append(_to_bytes(each))
 
         for each in self.row_ranges:
             r_kwrags = each.get_range_kwargs()