XAML Binding error, cannot find source

Hello,

I do get a binding error when trying to bind a string from codebehind as content of a ComboBoxItem in CompositeCollection.

<ComboBox.ItemsSource>
    <CompositeCollection>
        <ComboBoxItem Visibility="Collapsed" Content="{Binding Test, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:MultiSelectionDropdown}}}"/>
        <CollectionContainer Collection="{Binding Source={StaticResource ResourceKey=Pipeline}}"/>
    </CompositeCollection>
</ComboBox.ItemsSource>

I am trying to bind Test.