diff --git a/csst_common/io.py b/csst_common/io.py
index 79c6296de03ba73eb6dad278d67a340092ddcefc..edb2369f12127d0c9eadb715ea4af0a033b56900 100644
--- a/csst_common/io.py
+++ b/csst_common/io.py
@@ -97,7 +97,7 @@ def reformat_header(h: fits.Header, strip=True, comment=None):
     h_copy.remove("HISTORY", ignore_missing=True, remove_all=True)
     # add new comment
     assert len(h.cards) > 0
-    first_key = list(h.keys())[0]
+    first_key = list(h_copy.keys())[0]
     h_copy.add_comment("=" * 72, before=first_key)
     h_copy.add_comment(comment, before=first_key)
     h_copy.add_comment("=" * 72, before=first_key)