ARIMA fitted model gives NULL

I am trying to plot the residuals vs. the fitted values, but when I use the fitted function on my ARMA model, the output I receive is NULL. The data I am using is 500 values between roughly -5 and 5. The data should be modelled well by an ARMA(1,1) process.

I am not sure what the problem is in the following code.

model <- arima(data$Z, order = c(1,0,1), include.mean=FALSE)
fitted(model)