[R] logistic growth model

[Renalda]

I want to Fit a logistic growth model for
y = k *eb0+b1(age)/1 + eb0+b1(age), can some one help on how to get the 
initial coefficients b0 and b1? I need to estimate in order to do the 
regression analysis. When I run using b0=0.5 and b1=3.4818, I get the 
following error

397443.8 :  0.5 3.4818
Error in nls(Height ~ k * exp(b1 + b2 * Age)/(1 + exp(b1 + b2 * Age)),  :
   singular gradient
"please tell me what is wrong with my initials values, and how to get 
the initial values"