反射C#激活器返回空值

我不知道我做错了什么，当我运行此代码时，我得到一个异常：值不能为null。。。。当我在调试模式下运行它时，我看到"calculatorInstance"变量为null。请帮帮我。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Reflection;
namespace ReflectionWithLateBinding
{
    public class Program
    {
        static void Main()
        {
            //load the current executing assembly
            Assembly executingAssembly = Assembly.GetExecutingAssembly();
            //load and instantiate the class dynamically at runtime - "Calculator class"
            Type calculatorType = executingAssembly.GetType("ReflectionWithLateBinding.Calculator");

            //Create an instance of the type --"Calculator class"
            object calculatorInstance = Activator.CreateInstance(calculatorType);
            //Get the info of the method to be executed in the class
            MethodInfo sumArrayMethod = calculatorType.GetMethod("SumNumbers");
            object[] arrayParams = new object[2];
            arrayParams[0] = 5;
            arrayParams[1] = 8;
            int sum;
            sum = (int)sumArrayMethod.Invoke(calculatorInstance, arrayParams);
            Console.WriteLine("Sum = {0}",  sum);
            Console.ReadLine();
        }

        public class Calculator
        {
            public int SumNumbers(int input1, int input2)
            {
                return input1 + input2;
            }
        }
    }
}