The ArgumentOutOfRangeException Error my C#code has : I am learning C# these days and still a beginner.

While practicing creating methods, 
I made this method that yield the sum of the members of a numeric type's array.
(my codes are yet massive.. And I am not good at math as well..), 
VS says it has The ArgumentOutOfRangeException Error.
But, I don't know how to fix this. 
Please save me!


      public static int SumAll(int[] a)
        {
            List<int> sum = new List<int>();
            int sumAll;



            if (a.Length == 0)
            {
                sumAll = 0;
            }
            else
            {
                sum[0] = a[0];


                for (int i = 1; i < (a.Length - 1); i++)
                {
                    sum[i] = sum[i - 1] + a[i];
                }

                sumAll = sum[a.Length - 1];
            }
            return sumAll;  
        }

