The following warning message occurs when running the nls on some data:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Warning message:
In is.na(wts) : is.na() applied to non-(list or vector) of type 'NULL'
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and I would like to know what is causing it and how I can fix it.

As an example, the following, from Venables and Ripley, which does not
produce the warning message and then on the data which does procude the
warning message.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

##Venables and rippley

library(MASS)
data(wtloss)
wtloss.st = c(b0=90, b1=95, th=120)
n1<-length(wtloss)
Weight=c(1,rep(0.1,n1-1))

wtloss.fm = nls(Weight ~ b0 + b1*2^(-Days/th), data=wtloss, start=wtloss.st,
trace=T, weights=Weight )
wtloss.fm

##My data
y<-c(73,73,70,74,75,115,105,107,124,107,116,125,102,144,178,149,177,124,157,128,
169,165,186,152,181,139,173,151,138,181,152,188,173,196,180,171,188,174,198,172,
176,162,188,182,182,141,191,190,159,170,163,197)

x<-c(1,1,1,1,1,2,2,3,3,3,3,3,3,4,4,4,5,5,5,5,6,6,6,6,6,6,7,7,7,7,7,7,7,7,7,8,8,8,
8,8,8,8,8,8,8,9,9,9,9,9,11,12)

Data<-data.frame(cbind(y,x))

n2<-length(x)
weight<-c(1,rep(0.1,n2-1))

Ta<-min(Data$x)
Tb<-max(Data$x)
G.st<-c(k=0.005, g1=50,g2=550)


#no weights
growthFit<-nls(y~((g1)*exp((log(g2/g1))*(1-exp(-k*(x-Ta)))
  /(1-exp(-k*(Tb-Ta))))), data=Data, start=G.st, trace=T)


#with weights
growthFit.wts<-nls(y~((g1)*exp((log(g2/g1))*(1-exp(-k*(x-Ta)))
  /(1-exp(-k*(Tb-Ta))))), data=Data, start=G.st, trace=T, weights=weight)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Thanks in advance

Lisa

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