Event Weight in RooFit


I am sure this is a trivial question, but I can’t find a solution to it elsewhere:

I can’t seem to apply event weights in a RooDataSet. In reality I am reading from a TTree three values - x,y, and weight (and doing some manipulation before adding them to the RooDataSet) - but it seems they all arrive with a weight of 1.

Here’s a much simplified piece of code that replicates the problem:

    #include <iostream>
    #include "RooRealVar.h"
    #include "RooDataSet.h"

    void test(){
      RooRealVar x("x","x",0,100);
      RooRealVar y("y","y",20,80);
      RooDataSet s("s","s",RooArgSet(x,y));


      s.add(RooArgSet(x,y),0.5);  // give the event weight=0.5


      std::cout<<"s.sumEntries() = "<<s.sumEntries() <<"     weighted? "<<(s.isWeighted() ? "yes" : "no" )<<std::endl;

and the output from running this

    DataStore s (s)
      Contains 1 entries
        1)  x = 87  L(0 - 100)  "x"
        2)  y = 23.1416  L(20 - 80)  "y"
    s.sumEntries() = 1     weighted? no

I was expecting the sumEntries to contain the weight I assignment (0.5), and for it to say “weighted? yes” based on this member function of RooDataSet:
virtual void add(const RooArgSet& row, Double_t weight = 1.0, Double_t weightError = 0)

Thank you