Programmicaly add route

I write a simple utility that adds a route for specific interface. Code is very simple:

using System;
using System.Diagnostics;
using System.Net.NetworkInformation;
using System.Text;

class Program
{
    static void Main(string[] args)
    {
        const string firstConnection = "xxxx.yyyyy";
        const string routeAddMask = "route add XX.XX.XX.XX mask 255.255.255.255 XXX.XXX.XXX.XXX METRIC 99 IF {0}";
        StartProcess("rasdial", firstConnection);
        var interfaceId = GetInterfaceId(firstConnection);
        string routeAdd = string.Format(routeAddMask, interfaceId);
        StartProcess("route", routeAdd);
    }

    private static int GetInterfaceId(string name)
    {
        NetworkInterface[] adapters = NetworkInterface.GetAllNetworkInterfaces();
        var adapter = Array.Find(adapters, ni => ni.Name == name);
        var props = adapter.GetIPProperties().GetIPv4Properties();
        return props.Index;
    }

    private static void StartProcess(string name, string args)
    {
        var process = new Process
                   {
                       StartInfo = new ProcessStartInfo(name, args)
                                   {
                                       UseShellExecute = false,
                                       RedirectStandardOutput = true,
                                       StandardOutputEncoding = Encoding.ASCII
                                   }
                   };
        process.Start();
        while (!process.HasExited)
        {
            Console.WriteLine(process.StandardOutput.ReadToEnd());
        }
        process.WaitForExit();
        Console.WriteLine("Process {0} finished, exit code = {1}", name, process.ExitCode);
    }
}

rasdial works fine and I get interface number, but when I'm running route I get an error:

Invalid MASK generates an error, that is when (DEST & MASK) != DEST.

I think problem is that process start args are not passed because I get the same error while sending null instead of routeAdd parameter meanwhile this command works fine when i'm prompting it in cmd.

Executable is running as Administrator.