diff --git a/tohu/v6/spawn_mapping.py b/tohu/v6/spawn_mapping.py
index <HASH>..<HASH> 100644
--- a/tohu/v6/spawn_mapping.py
+++ b/tohu/v6/spawn_mapping.py
@@ -18,5 +18,5 @@ class SpawnMapping:
         try:
             return self.mapping[g]
         except KeyError:
-            logger.warning(f"Generator does not occur in spawn mapping: {g}")
+            logger.debug(f"Generator does not occur in spawn mapping: {g}")
             return g
\ No newline at end of file
Demote warning to debugging message because this case is expected