[R] nls problem? Error message, Why so?

[dave qwertyu]

Dear R's:

Thanks for the responses to my question on how to
start a nonlinear square fitting.  Now I encountered
another problem and wonder if I did anything wrong or
the algorithm by R has some difficulty?

My data:

p	theta
0.0	0.5
45.58	0.49
96.9	0.46
150.0	0.42
204.0	0.38
266.7	0.34
346.4	0.3
458.3	0.26
635.9	0.22
979.8	0.18
1990.0	0.14
7998.0	0.11


Then I did:

attach(mydata)
library(nls)
model <- nls( theta ~ thetar + (thetas - thetar)/(1 +
(alfa * p)^n)^(1-1/n),
                  data = mydata, 
                  start = list( thetar = 0.065, thetas
= 0.41, alfa = 0.075, n = 1.89))

I got this error message:

Error in numericDeriv(form[[3]], names(ind), env) : 
        Missing value or an Infinity produced when
evaluating the model


The data and initial guess should be sound and I was
able to fit the model with some fortran program.  I
just wonder how to perform the analysis by using R?

Any insights/comments, thanks a lot in advance!

Dave


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