Dont't call instant(None)

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":