Problem with PictureBox.Picture = NULL???

I don't understand why the code NULL cannot be used with picturebox.picture.

VB Code:

If (picPhoto.Picture = Null) Then
            If (Errfirst = "") Then
                Errfirst = "Picture "
            Else
                Errfirst = Errfirst & "& Picture "
            End If
        End If

What code should I replace the NULL

Replace it with 0 (that's a zero)