diff --git a/pyreduce/instruments/common.py b/pyreduce/instruments/common.py
index a56f9edf..67ac9239 100644
--- a/pyreduce/instruments/common.py
+++ b/pyreduce/instruments/common.py
@@ -227,7 +227,8 @@ def load_fits(
             extension = self.get_extension(h_prime, mode)
 
         header = hdu[extension].header
-        header.extend(h_prime, strip=False)
+        if extension != 0:
+            header.extend(h_prime, strip=False)
         header = self.add_header_info(header, mode)
         header["e_input"] = (os.path.basename(fname), "Original input filename")