While Loop Object Error

I am attempting to point excel at a named cell, fill that cell with a number, and then select the cell to the right of the active cell in the same row, and fill that cell with number + 1, until the loop is closed. When my Macro runs, I receive the "Object Required" error message.

Sub Populate()
    Dim Counter As Integer
    Counter = 1
    While Counter < EndingMonth.Value
        If Counter = 1 Then ActiveCell.Range(FirstMonthHeading).Select Else _ ActiveCell.Select
        ActiveCell.FormulaR1C1 = Counter
        ActiveCell.Offset(0, 1).Range(FirstMonthHeading).Select
        Counter = Counter + 1
    Wend
    
        
    
End Sub

Greatly appreciate any help.