[AccessD] which is the difference between...

hi all,
[paratiriseis] is an unbound text like:
=[paratirisi_1_tr]+Chr(13)+Chr(10) & [paratirisi_2_tr]+Chr(13)+Chr(10) & 
[paratirisi_txt]+Chr(13)+Chr(10) & [paratirisi_txt1]
and parat_lbl just a label

so I wonder why the first works while the other two are not!
-----------------------------------
If IsNull(Me.paratiriseis) Then
   Me.parat_lbl.Visible = False
else
...
end if
-----------------------------------
If Me.paratiriseis = 0 Then
   Me.parat_lbl.Visible = False
else
...
end if
-----------------------------------
If Len(Nz(Me.paratiriseis)) = null Then
   Me.parat_lbl.Visible = False
else
...
end if
-----------------------------------

thank you
/kostas