question about nls

Hi,all:
I met a problem of nls.

My data:
x    y
60 0.8
80 6.5
100 20.5
120 45.9

I want to fit exp curve of data.

My code:

nls(y ~ exp(a + b*x)+d,start=list(a=0,b=0,d=1))

Error in nlsModel(formula, mf, start, wts) :
  singular gradient matrix at initial parameter estimates

I can't find out the reason for the error.
Any suggesions are welcome.

The gradient is singular at your starting value so you will have to
use a better starting value.  If d = 0 then its linear in log(y) so
you can compute a starting value using lm like this:

lm1 <- lm(log(y) ~ x, DF)
st <- setNames(c(coef(lm1), 0), c("a", "b", "d"))

Also note that you are trying to fit a model with 3 parameters to only
4 data points.

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