NULL in LINQ variable

In the code below, I am populating a LINQ. From all the theoretical knowledge I have LINQ are not able to return NULL and this is the same in my case also. LINQ is a not null object but the associated variables are returning NULL values. I am quite sure that I am missing something in the code. Please check and revert.

private static readonly List<long> Primes = new List<long>() { 2, 3, 5, 7};
static void Main(string[] args)
    int Divisors;
    string input = "";
    bool start = true;
    while (!int.TryParse(input, out Divisors))
        if(!start) Console.WriteLine("Enter number");
        input = Console.ReadLine();
        start = false;

    int index = -1;
    while (index == -1)
        index = Primes.FindIndex(n => n > Divisors);
        if(index == -1) AppendPrime();
     List<int> cntPrimes = Primes.Select(n=>0) as List<int>;

While debugging, on the last line, the watch details are shown as 

Enumerable.Select() returned: Count = 12       cntPrimes: null