What does it mean by stack overflow exception and how can i fix my code?

I am trying to create a merge sort in to visual basic and keep getting the stack overflow error. I have no idea of how to fix this error and would like some help. Here is my code:

    Sub Main()
        Dim items() As String = {"Florida", "California", "Alabama", "Delaware", "Georgia"}
        Dim items2() As String = {"Florida", "California", "Alabama", "Delaware", "Georgia"}
        Mergesort(items, items2, 0, UBound(items))
    End Sub

    Sub Mergesort(ByVal items() As String, ByVal items2() As String, ByVal low As Integer, ByVal high As Integer)
        If low = high Then
            Return
        End If
        error on this line:
        Dim mid As Integer = (low + high) / 2

        Mergesort(items, items2, low, mid)
        Mergesort(items, items2, mid + 1, high)
        Merge(items, items2, low, mid, high)
    End Sub


'''
here is the error:

*System.StackOverflowException HResult=0x800703E9 Message=Exception of type'System.StackOverflowException' was thrown.*

I am trying to get the output to be the ordered list to be in alphabetical order:
["Alabama","California","Delaware","Florida","Georgia"}

There is a built in method to do what your want.

Private Sub OPCode2()
    Dim items() As String = {"Florida", "California", "Alabama", "Delaware", "Georgia"}
    Array.Sort(items)
    For Each i In items
        Debug.Print(i)
    Next
End Sub

Immediate Window result

Alabama
California
Delaware
Florida
Georgia