Bind C function and integrate with RooIntegrator1D


i am trying to numerical integrate a C function using RooFit. Somehow I got stuck in the compilation of my code and I cannot solve the compilation error:
My code (simplified):

int numIntBindFunc()
    RooRealVar x("x","x",-3,3) ;
    RooAbsReal* integralFct = bindFunction("bfunc",toThePower,x) ;

    RooIntegrator1D numInt1D(integralFct,.5,1.5,RooIntegrator1D::Trapezoid,1000,1e-3);
    Double_t result = numInt1D.integral();

    cout << " THE INTEGRAL IS: " << result << std::endl;
    return 0;

Double_t toThePower(Double_t x)
    return x*x;

The error message at the compilation:

Maybe someone has an idea and could help me out!
I am using ROOT 5.28/00a and RooFit v3.14.

Thanks very much,