IEnumerable gives error exception handeling can't read list

Example.cs

namespace ListExample
{
    using System.Collections.Generic;

    public class Program
    {
        private static readonly List<Animal> AnimalsToCheck = new List<Animal>
		{
			new Animal { Eater = Eater.Carnivore, Format = 1 },
			new Animal { Eater = Eater.Herbivore, Format = 3 }
		};

        private static readonly List<Wagon> Wagons = new List<Wagon>
		{
			new Wagon(
				1,
				5,
				1,
				new List<Animal>
				{
					new Animal { Eater = Eater.Carnivore, Format = 5 },
					new Animal { Eater = Eater.Herbivore, Format = 3 }
				}),
			new Wagon(
				2,
				9,
				5,
				new List<Animal>
				{
					new Animal { Eater = Eater.Carnivore, Format = 1 }
				})
		};

        public static void Checker(List<Animal> listAnimals)
        {
            foreach (var animal in listAnimals)
            {
                // This is needed to not get an endless loop (Because the length of the list
                // increases after each time the Add() method is called to Wagons).
                var wagonCount = Wagons.Count;

                for (var i = 0; i < wagonCount; i++)
                {
                    var wagon = Wagons[i];

                    // This is needed to not get an endless loop (Because the length of the list
                    // increases after each time the Add() method is called to wagon.Animals).
                    var animalCount = wagon.Animals.Count;

                    for (var j = 0; j < animalCount; j++)
                    {
                        var wagonAnimal = wagon.Animals[j];

                        if (wagon.StartCapacity <= wagon.MaxCapacity
                            && animal.Format + wagon.StartCapacity <= wagon.MaxCapacity
                            && wagonAnimal.Eater == Eater.Carnivore && animal.Eater == Eater.Herbivore
                            && animal.Format >= wagonAnimal.Format)
                        {
                            wagon.Animals.Add(animal);
                            Wagons.Add(wagon);
                        }
                        else
                        {
                            var newWagon = new Wagon();
                            newWagon.Animals.Add(animal);
                            Wagons.Add(newWagon);
                        }
                    }
                }

                var wag = new Wagon();
                wag.Animals.Add(animal);
                Wagons.Add(wag);
            }
        }

        public static void Main()
        {
            Checker(AnimalsToCheck);
        }
    }

    public class Animal
    {
        public Eater Eater { get; set; }

        public int Format { get; set; }
    }

    public class Wagon
    {
        public List<Animal> Animals = new List<Animal>();

        public Wagon()
        {
        }

        public Wagon(int startCapacity, int maxCapacity, int format, List<Animal> animals)
        {
            this.StartCapacity = startCapacity;
            this.MaxCapacity = maxCapacity;
            this.Format = format;
            this.Animals = animals;
        }

        public int Format { get; set; }

        public int MaxCapacity { get; set; }

        public int StartCapacity { get; set; }
    }

    public enum Eater
    {
        Carnivore,

        Herbivore
    }
}