diff --git a/src/osmo_gsm_tester/config.py b/src/osmo_gsm_tester/config.py
index 7f1e52f..87b3da4 100644
--- a/src/osmo_gsm_tester/config.py
+++ b/src/osmo_gsm_tester/config.py
@@ -176,6 +176,8 @@
     return yaml.dump(config, default_flow_style=False)
 
 def _standardize_item(item):
+    if item is None:
+        return None
     if isinstance(item, (tuple, list)):
         return [_standardize_item(i) for i in item]
     if isinstance(item, dict):