Setting Parameter Value to Null

in TSQLQuery I can set NULL-Values to any Parameter
TSQLQuery(aDataset).ParamByName(aParamName).datatype := ftUnknown;
TSQLQuery(aDataset).ParamByName(aParamName).Value := aValue; // could be null
everthing works well
with TXDataset
TXDataset(aDataset).ParamByName(aParamName).datatype := ftUnknown;
TXDataset(aDataset).ParamByName(aParamName).Value := aValue; // could be null
I get an Error 'Fieldtype is unknown if aValue is NULL

Yes, you have to specify the data type of every parameter.