PIEBALDconsult GetRepeats with count has ArgumentNullException error, C#

Language: Layout:

using System;
using System.Collections.Generic;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var li = new ListIndex<Car>(new Car[] 
            {
                new Car() { Brand="Mazda", Model="CX9",Year=2016 },
                new Car() {Brand="Ford", Model="XR5",Year=2014 },
                new Car() {Brand="Ford", Model="XR5",Year=2010 },
                new Car() {Brand="Holden", Model="Commodore",Year=2015 },
                new Car() { Brand="Mazda", Model="CX9",Year=2016 },
                null
            });

foreach (Tuple<Car, IList<int>> dup in li.Duplicates)
                Console.WriteLine("{0} appears {1} times", dup.Item1, dup.Item2.Count);
            Console.WriteLine("\r\n-- done!");
        }
    }
    
    public class ListIndex<T>
    {
        private readonly Dictionary<T, List<int>> map;

public ListIndex (ICollection<T> Input)
        {
            map = new Dictionary<T, List<int>>();
            if (Input != null)
            {
                int i = 0;
                foreach (T t in Input)
                {
                    if (!map.ContainsKey(t)) map[t] = new List<int>();
                    map[t].Add(i);
                    i++;
                }
            }

return;
        }

public IEnumerable<Tuple<T, IList<int>>> Duplicates
        {
            get
            {
                foreach (KeyValuePair<T, List<int>> kvp in map)
                    if (kvp.Value.Count > 1) yield return new Tuple<T, IList<int>>(kvp.Key,kvp.Value.AsReadOnly());
                yield break;
            }
        }
    }

public class Car : IEquatable<Car>
    {
        public string Brand { get; set; }
        public string Model { get; set; }
        public int Year { get; set; }

public override string ToString()
        {
            return string.Format("{0} {1} {2}", Year, Brand, Model);
        }

public override int GetHashCode()
        {
            int hash = 17;
            hash = hash * 31 + Brand.GetHashCode();
            hash = hash * 31 + Model.GetHashCode();
            hash = hash * 31 + Year.GetHashCode();
            return hash;
        }

public override bool Equals(object o)
        {
            return o == null ? false : Equals(o as Car);
        }

public bool Equals(Car car)
        {
            return Brand == car.Brand && Model == car.Model && Year == car.Year;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,12 sec, absolute running time: 0,11 sec, cpu time: 0,11 sec, average memory usage: 13 Mb, average nr of threads: 3