diff --git a/src/gentropy/config.py b/src/gentropy/config.py
index 1d864274f..2536f6c6e 100644
--- a/src/gentropy/config.py
+++ b/src/gentropy/config.py
@@ -17,7 +17,7 @@ class SessionConfig:
     write_mode: str = "errorifexists"
     spark_uri: str = "local[*]"
     hail_home: str = os.path.dirname(hail_location)
-    extended_spark_conf: dict[str, str] | None = None
+    extended_spark_conf: dict[str, str] | None = field(default_factory=dict[str, str])
     _target_: str = "gentropy.common.session.Session"