Dear Team,
I'm trying to fit modified Richards equation to simulated cumulated volume (v_elong) data over time (dia).
Parameters to be estimated are Maximum volume (Vol_max), flexibility parameter (V), maximum rate (Max_rate) and lag phase (lamda)
The used code and error are:

PHP Code:
 nl ( V_elong= {Vol_max=80}*(  1 + ( {V}*exp(1+ {V})*({Max_rate}*(1+{V})*(1+(1/{V}))*({lamda}- dia ) / {Vol_max}) )^(-1/{V})) )
(obs = 31)

starting values invalid or some RHS variables have missing values
r(480); 
I was trying with a lot of initial values for V with no result. I have also fit logistic and 3-parameters Gompertz models with no problems.
Could you be so kind to advice me?
Thank you in advance,
Jorge