WPF - Binding a ComboBoxItem visibility using BooleanToVisibilityConverter - wpf

Question

I have a Datagrid, with a column that has a combobox in its header. I want to control the visibility of items in the combobox based on the value of a certain flag.
This is how my XAML looks:
<DataGrid x:Name="Table1" Height="{Binding ElementName=ElasticOne1,Path=ActualHeight}" Width="{Binding ElementName=ElasticOne1,Path=ActualWidth}" Padding="5,5,5,5" IsReadOnly="True"
HorizontalAlignment="Stretch" VerticalAlignment="Stretch" SelectionMode="Single" >
<DataGrid.Columns>
<DataGridTextColumn x:Name="Column2" Header="Source" Width="80">
<DataGridTextColumn.HeaderTemplate>
<DataTemplate>
<StackPanel Margin="0">
<StackPanel.Resources>
<BooleanToVisibilityConverter x:Key="BoolToVis"/>
</StackPanel.Resources>
<TextBlock Text="{Binding Content, RelativeSource={RelativeSource Mode=TemplatedParent}}" Margin="5"/>
<ComboBox HorizontalAlignment="Stretch" Margin="0" >
<ComboBoxItem Tag="0" IsSelected="True">All</ComboBoxItem>
<ComboBoxItem Visibility="{Binding ShowCopybookInSourceCombobox, Converter={StaticResource BoolToVis}}">Copybook</ComboBoxItem>
<ComboBoxItem>Prototype</ComboBoxItem>
</ComboBox>
</StackPanel>
</DataTemplate>
</DataGridTextColumn.HeaderTemplate>
</DataGridTextColumn>
</DataGrid.Columns>
And this is the declaration of the flag in code:
Public ReadOnly Property ShowCopybookInSourceCombobox As Boolean
Get
Return myFlag1 Or myFlag2
End Get
End Property
No matter what the value of my flag, the combobox item is always showing. I haven't seen any examples so far of binding the visibility of a comboboxitem this way. Am I on the wrong path?
Thanks for all your input.
Edit: BooleanToVisibilityConverter is a built-in class that I am using - https://msdn.microsoft.com/en-us/library/system.windows.controls.booleantovisibilityconverter(v=vs.110).aspx