Check if Cells A2 and A3 are blank in Excel

Question

I tried this code:

public static void CellsBlank()
{

Worksheet worksheet;

Workbook wb = Global.Variables.GlobalVariables.oXL.ActiveWorkbook;

Excel.Range cell1 = (Excel.Range)worksheet.get_Range("A2", "A2");

Excel.Range cell2 = (Excel.Range)worksheet.get_Range("A3", "A3");
}

But that throws a debug error when it hits my Excel.Range cell1 line!  How should I re-write this?