RelativeSource FindAncestor with a Custom Business Object as the Ancestor Type

I have a Telerik GridView nested inside another GridView.

The outer GridView is bound to a collection of Order entities which is defined in my view model.

The inner GridView is bound to the OrderDetails navigation property (which is a collection of OrderDetail entities) of each Order entity.

In the inner GridView is a ComboBox. I'm trying to bind the ItemsSource of that ComboBox to a property which is kept in the OrderDetail object. However, visual studio is rejecting the xaml binding.

xmlns:bus="clr-namespace:StoreLibrary.DataObjects"
...
<ComboBox SelectedItem="{Binding Item, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" DisplayMemberPath="Description" Text="{Binding Item.Description}"
      ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type: bus:OrderDetail}}, Path=availableItems}" />

The error I get is this: The name "OrderDetail" does not exist in the namespace "clr-namespace:StoreLibrary.DataObjects"

But it does. That's exactly where the entity is defined.

Clearly something is wrong with my binding syntax. Or else my xaml namespace import at the top.

Thanks!