C# code to get maximum dpi of installed printer.

I want to get Maximum Dpi of installed or selected printer. I tried

PrinterSettings ps = new PrinterSettings();
var maxResolution = ps.PrinterResolutions.OfType<PrinterResolution>()
                                         .OrderByDescending(r => r.X)
                                         .ThenByDescending(r => r.Y)
                                         .First();
MessageBox.Show(String.Format("{0}x{1}", maxResolution.X, maxResolution.Y));

 
I got three outputs. The third is Canon iP1300 -1x-1. What is -1x-1. As it mentioned in specifications of printer the maximum dpi of Canon iP1300 is 4800x1200. The output must be Canon iP1300 4800x1200 instead of -1x-1. Can anyone explain -1x-1 and why I am not getting the desired output?