System.IndexOutOfRangeError with merge sort

So I am working on merge sort in C# and I keep getting the IndexOutOfRangeException error on a piece of code. I am trying to debug and it looks like it has something to do with left[i]. This is a list of a seperate SortList Class.

The following code throws the error:

        if (left.Count < mid)
        {
            for (int i = 0; i < mid; i++)
            {
                left[i] = list[i];
            }
            for (int i = mid; i < list.Count; i++)
            {
                right[i] = list[i];
            }
        }

The following error is thrown:

System.IndexOutOfRangeException
  HResult=0x80131508
  Message=De index ligt buiten de matrixgrenzen.
  Source=2-Sorting
  StackTrace:
   at ALGA.SortList.set_Item(Int32 index, Int32 value) in C:\Users\Tristan\Documents\School\GitHub repositories\week-2-sorting-tommeren\2-Sorting\SortList.cs:line 84
   at ALGA.Mergesort.mergesort(ISortList list) in C:\Users\Tristan\Documents\School\GitHub repositories\week-2-sorting-tommeren\2-Sorting\Mergesort.cs:line 26
   at ALGA_test.MergesortTest.MergesortEfficiently() in C:\Users\Tristan\Documents\School\GitHub repositories\week-2-sorting-tommeren\2-Sorting-Test\MergesortTest.cs:line 53

Thank you in advance.

So the error is gone while initializing the SortList but the debug information doesn't give me any info on left or right, while I iterated through it multiple times. Do you know why?