[R] Generating a best fit line for non linear data

[BornSurvivor]

I have the following data set, and I have to find the line of best fit using
this equation, 
y = a*(1 - exp(-b*x)).

samples = seq(1,20,by=1)
species = c(5,8,9,9,11,11,12,15,17,19,20,20,21,23,23,25,25,27,27,27)
plot(samples,species, main = "Accumulation Curve for Tree Species Richness",
xlab = "Samples", ylab = "Number of Species")
curve((y = 27*(1 - exp(-.15*x))),from=0,to=20,add = T)

I tried to find the best fit curve using:

fit = nls(species ~ a *(1 - exp(-b*x)),start = list(a = 27, b = .15)

but I get a "parameters without starting value in 'data': x" and I don't
have any idea what this means, or how to fix the above code.  

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