In ROOT, before you try to fit your graph or histogram, you MUST set “reasonable” initial values for ALL parameters of your function (except for some “built-in” formulas, like “expo”, 1-D and 2-D “landau”, 1-D and 2-D “gaus”, “chebyshev”, “pol”, for which the standard fit procedure can automatically “guess” them). Otherwise, the fitting procedure may easily misbehave.