I had to add an extra assignment to make my code work, does anyone know why?

Working code:

Dim tag_dic As Object
Set tag_dic = CreateObject("Scripting.Dictionary")
Dim apa As String

If rs1.RecordCount > 0 Then
    rs1.MoveFirst
    Do While Not rs1.EOF
        apa = rs1!Tag
        If (tag_dic.Exists(apa)) Then
            With rsdup
                .AddNew
                rsdup!Tag = apa
                rsdup!ID = (equ   1)
                .Update
                .Bookmark = .LastModified
            End With
        Else
            tag_dic.Add apa, "a"
        End If
            equ = equ   1
        End If
        rs1.MoveNext
    Loop
End If

If I take out the dummy variable "apa", and instead use the field "rs1!Tag" directly, the code doesn't work and everything gets added to the rsdup record set. How does field evaluation work in a loop?