RooMoment doens't apply sqrt

Roofit and RooStats
1

Dear expert, why I am getting a parabola in this case? It seems the root to be not applied

import ROOT
x = ROOT.RooRealVar('x', 'x', -10, 10)
mean = ROOT.RooRealVar('mean', 'mean', 1, -1, 1)
sigma = ROOT.RooRealVar('sigma', 'sigma', 0.1, 0.1, 1)
gaus = ROOT.RooGaussian("gaus", "gaus", x, mean, sigma)
frame = sigma.frame()
#s = gaus.moment(x, 2, True, True)
s = ROOT.RooMoment("s", "s", gaus, x, 2, True, True)
s.plotOn(frame)
frame.Draw()