What is the lifetime of a global variable in excel vba?

I've got a workbook that declares a global variable that is intended to hold a COM object. ``` Global obj As Object ``` I initalize it in the Workbook_Open event like so: ``` Set obj = CreateObject("ComObject.ComObject"); ``` I can see it's created and at that time I can make some COM calls to it. On my sheet I have a bunch of cells that look like: ``` =Module.CallToComObject(....) ``` Inside the Module I have a function ``` Function CallToComObject(...) If obj Is Nothing Then CallToComObject= 0 Else Dim result As Double result = obj.GetCalculatedValue(...) CallToComObject= result End If End Function ``` I can see these work for a bit, but after a few sheet refreshes the obj object is no longer initialized, ie it is set to Nothing. Can someone explain what I should be looking for that can cause this?