Cannot select/use 4PL Rodbard curve fit

[#1]

Hi there,

I am trying to fit a dose-response curve to the Rodbard 4PL model. I have 3 columns, my Response, Regressor, and a weighting. The option for Rodbard is greyed out and I cannot select it. all other 4PL fits are available so I'm not sure why the one I need is not available? 

Kindly,

TK

[#2]

Could you have the same reason as here Why can't use 'Fit Logistic 4P Rodbard' ? ?

That model is disallowed when you have a dose equal to zero.