[R] help on "plotfit" function in package "nlstools"

[Yi Li]

I encountered this problem when I was learning nonlinear regression with R :
there is this function "plotfit" in package "nlstools",which "displays
a superimposed plot of the dependent variable versus one the
independent variables together with the fitted model.",but when I
execute the example given in the help page, it keeps saying that :

> plotfit(nls1, smooth = TRUE)
Error in form[[3]] : object of type 'symbol' is not subsettable

can anybody tell me what this means and how to fix it ?