You should use income as a continuous / numerical variable for two reasons: 1.) If each income number in your data frame is treated as a level of a factor, you have a huge amount of different factors and this costs lots of degrees of freedoms (that means the proportion of data points to regression parameters is really bad), and 2.) you would not use all information available in this variable, such as order and proportions. So if you want to use income as a covariate (and that is what I understood) use it as a numerical predictor. And you can possibly even improve your model by centring this predictor.