UDF Returns #Value! Error After Opening Another Workbook

Having a problem with a simple udf to calculate values across multiple sheets using the calling cell as a reference point. It works great until you open another workbook and then switch back. At that point all cells calling this udf return a #value! error. If I type anywhere on the sheet all of the values will recalculate but this is less than ideal for the end users here. below is example of concept

Function TestMe()
Application.Volatile (True)
Dim strCallAddress As String
Dim intSubTotal As Integer
strCallAddress = Application.Caller.Address
intSubTotal = Worksheets(2).Range(strCallAddress).Value
intSubTotal = intSubTotal + Worksheets(3).Range(strCallAddress).Value
TestMe = intSubtotal
End Function