Wrong sum calculation of objects in array - vb.net

Question

I received code with an object (we'll call it Obj) with a property named Number. An array of these objects is defined with 40 objects in indexes 0..39. I don't need object at index 0.
The code has a static variable saving the sum of all Number's of Obj's in the array. This variable is initialized only once. Right after the Number values themselves are defined.
It usually works, but sometimes I log an exception (which I haven't been able to reproduce) with values of different objects and I see there the value of NumberAll is wrong (I've recently logged 1722 and 2134. Not that it should matter - but the only common prime factor for them is 2. should be 1554, if you were wondering, you can check me below :) ).
EDIT: The exception is an Index was outside the bounds of the array on the loop brought in the code below - a direct effect of this issue.
I've tried two methods of summing, brought below.
Relevant code follows. There is nowhere else in the code that a value is assigned to NumberAll.
Static NotFirstTime As Boolean, NumberAll As Integer
If NotFirstTime = False Then GoTo Data
sPoint: 'Code and more code
' ...
' ...
breakValue = someValue Mod NumberAll
Sum = 0
I = 1
Try
Do
If Sum + myArray(I).Number >= breakValue Then
Exit Do
End If
Sum = Sum + myArray(I).Number
I = I + 1
Loop
Catch ex As Exception
'log error and variable data - breakValue is larger than Sum will ever get
'hence the loop reaches the point of trying myArray(40) which doesn't exist
End Try
' ...
Data:
NotFirstTime = True
myArray(1).Number = 37
myArray(2).Number = 34
myArray(3).Number = 44
myArray(4).Number = 31
myArray(5).Number = 59
myArray(6).Number = 26
myArray(7).Number = 33
myArray(8).Number = 28
myArray(9).Number = 20
myArray(10).Number = 13
myArray(11).Number = 92
myArray(12).Number = 65
myArray(13).Number = 71
myArray(14).Number = 22
myArray(15).Number = 22
myArray(16).Number = 42
myArray(17).Number = 26
myArray(18).Number = 26
myArray(19).Number = 33
myArray(20).Number = 34
myArray(21).Number = 22
myArray(22).Number = 19
myArray(23).Number = 85
myArray(24).Number = 72
myArray(25).Number = 47
myArray(26).Number = 40
myArray(27).Number = 47
myArray(28).Number = 54
myArray(29).Number = 48
myArray(30).Number = 44
myArray(31).Number = 37
myArray(32).Number = 34
myArray(33).Number = 44
myArray(34).Number = 9
myArray(35).Number = 57
myArray(36).Number = 37
myArray(37).Number = 19
myArray(38).Number = 13
myArray(39).Number = 68
NumberAll = 0
'Only one of the following methods is used. They seem to give the same result. The bug exists in both.
'Method one
For I = 1 To 39 'E מספר המסכתות
NumberAll = NumberAll + myArray(I).Number
Next I
'Method two
Dim myList As List(Of Obj) = New List(Of Obj)(myArray)
NumberAll = myList.Sum(Function(b) b.Number)
NumberAll -= myArray(0).Number
GoTo sPoint