Why is the Tag property on a selected TabPage null after being set earlier?

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VB

WinForms

I dynamically create a bunch of tabs for a TabStrip control, with code like

SubsTabControl.TabPages.Add(New TabPage() With {.Text = s.Id, .Tag = s})

yet when I try and access the Tag property in subsequent code, it is null? Why is this? The line above is in GenerateTabs, the null reference occurs here:

GenerateTabs()

If (SubsTabControl.TabCount > 0) Then
    With SubsTabControl.SelectedTab
        Dim tab = SubsTabControl.TabPages.Item(SubsTabControl.SelectedIndex)
        Dim subProcess As SubProcess = DirectCast(.Tag, SubProcess)
        .Controls.Clear()
        .Controls.Add(GridControl)
        GridControl.ProcessDesc = subProcess.Process.Desc
        GridControl.SubId = subProcess.Id
    End With
End If