diff --git a/km3io/offline.py b/km3io/offline.py
index cb68d2d78afcb256ef92ff4b6c40e575a7718640..faad4f56e92f30be577b7a727cc8179d1ab8e489 100644
--- a/km3io/offline.py
+++ b/km3io/offline.py
@@ -189,7 +189,7 @@ class Header:
     """The header"""
     def __init__(self, header):
         self._data = {}
-        self._missing_keys = list(set(header.keys()) - set(mc_header.keys()))
+        self._missing_keys = set(header.keys()) - set(mc_header.keys())
 
         for attribute, fields in mc_header.items():
             values = header.get(attribute, '').split()