[R] logistic growth model

[magushi]

HI I want to Fit a logistic growth model for this data set where
y = k*exp(b0+b1(age))/1 + exp(bo+b1(age)),
start = list(b0 = 3, b1 = 3.5 ),
                       trace = TRUE)

I need to find the initial valued for b0 and b1. K =3. When I run using b0=3
abd b1 = 3.5, or any number I get the following error


397448.4 :  3.0 3.5 
Error in numericDeriv(form[[3L]], names(ind), env) : 
  Missing value or an infinity produced when evaluating the model. Can
anyone help me how to get the initial values? what does this error msg
implies?
> 


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