While computing a variance on my dataset, I end up with: error C2677: binary '+' : no global operator found which takes type 'System::Object ^const ' (or there is no acceptable conversion)

The error message point to the line 65 of mean.hpp (see below piece of code). Can someone explain what's going wrong? Thanks

template<typename Args>            // line 59
    void operator ()(Args const &args) //line 60
    {                                  // line 61
        std::size_t cnt = count(args); // line 62
        this->mean = numeric::fdiv(    // line 63
            (this->mean * (cnt - 1)) + args[parameter::keyword<Tag>::get()] // line 64
          , cnt // line 65
        );      // line 66
    }           // line 67 

my code:

#pragma once
#include "stdAfx.h"
#include "math.h"
#include <boost/accumulators/accumulators.hpp>
#include <boost/accumulators/statistics/stats.hpp>
#include <boost/accumulators/statistics/variance.hpp>


.....

accumulator_set< double, stats<tag::variance> > acc_variance;
                  DataRow^i;

                 for (int i=1;i-1<dbdataset->Rows->Count-1;i++){
                     dbdataset->Rows[i-1]["DataRet"]=log(System::Convert::ToDouble
(dbdataset-       >Rows[i-1]["Pop"])/System::Convert::ToDouble(dbdataset->Rows[i]["Pop"]));
                     acc_variance(dbdataset->Rows[i-1]["DataRet"]);
                     lblvol->Text = Convert::ToString(sqrt(variance(acc_variance)));
                 }