从ItemsControl绑定到父DataTemplate属性

假设我有这个ViewModel和xaml:

class MyViewModel
{
    public MyStringValue {get;set;} = "HelloWorld"
    public IList<CustomObject> ChildViewModels{get;set;}
}
<DataTemplate DataType="{x:Type local:MyViewModel}">
    <ItemsControl ItemsSource="{Binding ChildViewModels}">
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <Grid>
                    <TextBlock Text="{Binding Path=MyStringValue,
                        RelativeSource={RelativeSource Mode=FindAncestor,
                        AncestorType={x:Type local:MyViewModel}}}"/>
                </Grid>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</DataTemplate>

我一直收到以下错误消息："找不到引用为"RelativeSource FindAncestor"的绑定的源…"所以基本上，我正在尝试绑定ItemsControl的parents属性容器，但似乎我做不到。