Can someone explain me why it throws an ArgumentOutOfRangeException?

Question

static List<object> JosephusPermutation(List<object> items, int k)
{
  List<object> solution = new List<object>();
  int index = k - 1;
  int len = items.Count;
  int count = 1;
  foreach(object i in items){
    if(index<len){
      solution.Add(items[index]);
      index += k;
    }
    else{
      index = (index - 1) - len * count;
      count++;
    }
  }
  return solution;
}

I tried to create a function that takes every k element in the array and get an OutOfRangeException and don't know why it's happening. Can you explain me why these piece of code behaves like these? Appreciate your help!