Table column sum generates error

I have a worksheet containing a table. The following function returns the total of any column.

Function ColSum(col As Integer)

    Dim lo As ListObject
    Dim TotalSP As Integer
    
    
    Application.Volatile
    
    Sheets("ContractAuth").Activate
    Set lo = ActiveSheet.ListObjects(1)
    ColSum = Application.WorksheetFunction.Sum(lo.DataBodyRange.Columns(col))
    
End Function

I find that often the cell holding the result of this function has the error '#VALUE' and I have to press F9 to force calculation, although automatic calculation is already on. This appears to happen when a change is made to another worksheet in the same workbook - I have to return to the worksheet containing the table and press F9.