Accessing BuiltInParameter.RBS_PIPE_FLUID_TEMPERATURE_PARAM in Revit 2018 fails

[xavier.atero]

I am trying to read the fluid temperature of a system using the following code:

// read MEPSystem in system variable
// ...
if (system is PipingSystem)
{
    PipingSystem piping_system = system as PipingSystem;
    PipeSystemType system_type = piping_system.SystemType;
    if (system_type == PipeSystemType.DomesticColdWater || system_type == PipeSystemType.DomesticHotWater || system_type == PipeSystemType.ReturnHydronic)
    {
        Parameter parameter__temperature_of_fluid = piping_system.get_Parameter(BuiltInParameter.RBS_PIPE_FLUID_TEMPERATURE_PARAM);
    }
}

Exceuting the code on Revit 2017:

• parameter__temperature_of_fluid = {Autodesk.Revit.DB.Parameter}

Exceuting the code on Revit 2018:

• parameter__temperature_of_fluid = null

I have tried with both 2017 and 2018 dlls and the result is the same; the OS is Windows 10. Any ideas? Thanks in advance.