Only loop selected rows in Excel VBA

I'm trying to setup a loop that only effects the selected rows. I'm probably just missing something simple but I'm at the point I need a second set of eyes on it.

I'm tying to have it do a simple loop to update column D if Column E is blank within the selected rows. Currently it gives me a type mismatch error.

Basically trying to make this:

Into this:

When I select all the rows for that selection of ID's. I'm assuming I'm missing something simple or my syntax is off a bit. Any help would be greatly appreciated!

My Code currently:

Function pages()
   Dim rng As Range
   Dim lRowSelected As Long
               
   For Each rng In Selection.Rows
      lRowSelected = rng.Row
      If IsNull(ActiveWorkbook.Worksheets(1).Range("D" & rng)) Then
         ActiveWorkbook.Worksheets(1).Range("D" & rng).Value = ActiveWorkbook.Worksheets(1).Range("D" & rng).Value + 1
      End If
   Next rng
        
End Function

Here you're checking for empty cells in Col D - did you mean to check Col E?

If IsNull(ActiveWorkbook.Worksheets(1).Range("D" & rng)) Then

Something like this is a little simpler:

Function pages()
   Dim c As Range
   
   For Each c In Selection.EntireRow.Columns("E").Cells
        If Len(c.Value) = 0 Then 
            With c.Offset(0, -1)    'updating Col D...
                .Value = .Value + 1
            End With
        End If
   Next c
        
End Function