diff --git a/leximpact_socio_fiscal_api/routers/simulations.py b/leximpact_socio_fiscal_api/routers/simulations.py
index c5dbed9653a1dcf85198fb6af97c679d5122bbac..6330fb7cf54a84fc0ed8c9ed330f10d7b56d9b1c 100644
--- a/leximpact_socio_fiscal_api/routers/simulations.py
+++ b/leximpact_socio_fiscal_api/routers/simulations.py
@@ -119,8 +119,8 @@ def apply_parametric_reform(parameters, parameter_change_by_name):
             changeType = change.get("type")
             if changeType == "parameter":
                 parameter.update(
-                    start=instant(change.get("start")),
-                    stop=instant(change.get("stop")),
+                    start=instant(change["start"]) if change.get("start") != None else None,
+                    stop=instant(change["stop"]) if change.get("stop") != None else None,
                     value=change.get("value"),
                 )
             elif changeType == "scale":