diff --git a/common/python/nomadcore/md_data_access/MDDataAccess.py b/common/python/nomadcore/md_data_access/MDDataAccess.py
index d762bc805aff5c48fde1eca2b7d1f5282c8c902e..36c76b87e16f666f22c24bff094d277768283960 100644
--- a/common/python/nomadcore/md_data_access/MDDataAccess.py
+++ b/common/python/nomadcore/md_data_access/MDDataAccess.py
@@ -957,6 +957,7 @@ class MDDataAccess(object):
             }
 
     def initialize(self):
+        self.natoms = None
         self.topology = None # Main storage for topology data
         self.trajectory = None # Main storage for trajectory data
         self.forcefield = None # Main storage for force field parameters