plot and lines with time series data

Dear all,

I am doing some time series analysis with R now. The problem is, when
I create a time series object using "ts" function, then after fitting
the model, the predicted values cannot be plotted with "ts" object
together using "lines". ie.

ts.series<-ts(x,start, end)
plot(ts.series)
lines(predict(fit.model))

This doesn't work.

Does anyone know about how to make it work ?

Thank you very much