Default y=0 line in RooPlot

Hi

When a RooPlot is drawn (with whatever content), it seems that by default always a black line centered at y=0 is drawn as well. How to get rid of it?

using namespace RooFit;
RooRealVar *invariant_mass = new RooRealVar("invariant_mass","invariant_mass",100.0,100.0,160.0);
RooPlot* frame=invariant_mass->frame(100,160);
frame->Draw();

Thanks
Jana