diff --git a/hed/models/base_input.py b/hed/models/base_input.py
index 9f437102..58bb94c9 100644
--- a/hed/models/base_input.py
+++ b/hed/models/base_input.py
@@ -137,7 +137,7 @@ def _indexed_dict_from_onsets(onsets):
 
     @staticmethod
     def _filter_by_index_list(original_series, indexed_dict):
-        new_series = pd.Series(["n/a"] * len(original_series))
+        new_series = pd.Series(["n/a"] * len(original_series), dtype=str)
 
         for onset, indices in indexed_dict.items():
             if indices: