diff --git a/common/python/nomadcore/md_data_access/MDDataAccess.py b/common/python/nomadcore/md_data_access/MDDataAccess.py
index d2104024f66f9ef3cdeb767c58dd73e81af00730..2ff3241ff74c06c81868f6ac2081374bdcb1951a 100644
--- a/common/python/nomadcore/md_data_access/MDDataAccess.py
+++ b/common/python/nomadcore/md_data_access/MDDataAccess.py
@@ -3730,13 +3730,14 @@ class MDDataAccess(object):
         """
 
         ioformat = None
-        _format = file_format.replace('-', '_')
         module_handler = None
-        module_name = ase_io.formats.format2modulename.get(file_format, _format)
-        try:
-            module_handler = ase_io.formats.import_module('ase.io.' + module_name)
-        except ImportError:
-            pass
+        if file_format:
+            _format = file_format.replace('-', '_')
+            module_name = ase_io.formats.format2modulename.get(file_format, _format)
+            try:
+                module_handler = ase_io.formats.import_module('ase.io.' + module_name)
+            except ImportError:
+                pass
         if module_handler:
             module_read = getattr(module_handler, 'read_' + _format, None)
             if module_read and not inspect.isgeneratorfunction(module_read):