New Post: Excel ActiveWorkbook null

I am trying to read the current Workbook in Excel.

I call:

Excel.Application excelApplication = Excel.Application.GetActiveInstance();
Excel.Workbook currentWorkBook = excelApplication.ActiveWorkbook;

currentWorkBook is null after these calls, even though I can physically see that a workbook is open in excel.

Do I need to make some call beforehand? Is this a known issue? Any information would help.

Thanks.