TMinuit->Contours() producing 0 contours


I am trying to make a contour plot using TMinuit. After the function minimization, I use the following commande:

where gMinuit is my TMinuit object, and par1 and par2 are the parameters for which I draw the contour plot. However, I always get at most 4 points in the contour plot (when it doesn’t fail), with the warning:

MINUIT WARNING IN MNContour ============== Cannot find midpoint, try closer. Warning in <TMinuit::Contour>: Returning a TGraph with 4 points only

What should I do to get a reasonable number of points?

Thank you in advance