plotting the result of a nonlinear regression

Hello,
to plot the result of a singular non linear regression (using nls) I usually use the function plotfit, for example:

r.PTG.V<-nls(PTG.P~ fz1(Portata, a,b), data=dati, start=list(a=10, b=10), nls.control(maxiter=200), algorithm='port',
trace=TRUE, na.action=na.omit, lower=list(a=0, b=10), upper=list(a=100, b=100))
plotfit(r.PTG.V)

I tried to use the function plotfit on the result of the following for loop but I got an error message:
for (i in 1:length(formList))
{
   resultList[[i]] <- nls(formList[[i]], data=subset(dati, Fiume=="Laveggio"), start=startList7[[i]],
   nls.control(maxiter=1000, warnOnly=TRUE), algorithm='port', na.action=na.omit,lower=lowerList7[[i]],
   upper=upperList7[[i]])
}
plotfit(resultList[[1]]

"Error in diff(as.numeric(y[ord])) :
 (list) object cannot be coerced to type 'double'"

Could somebody tell me what this error means? how can I fix this problem?
Any suggestion appreciated!!
Thanks,
Laura