diff --git a/mne/epochs.py b/mne/epochs.py
index 515bbb69a72..e6a1f684581 100644
--- a/mne/epochs.py
+++ b/mne/epochs.py
@@ -3232,7 +3232,7 @@ def _diff_input_strings_vs_event_id(input_strings, input_name, event_id):
 
     # keep_first and keep_last names
     start_idx = stop_idx
-    metadata[columns[start_idx:]] = ""
+    metadata[columns[start_idx:]] = None
 
     # We're all set, let's iterate over all events and fill in in the
     # respective cells in the metadata. We will subset this to include only