JPAContainer addNestedContainerProperty "Invalid property"

I have two entities and would show data from this in one table

In SQL I make this with the following SQL Statement:
Select werk.*, firma.* From werk Inner Join firma On werk.firmaID = firma.ID

public class TestJoinTables extends Window {
    private JPAContainer<Werk> werkContainer;
    public TestJoinTables() {
        werkContainer = JPAContainerFactory.make(Werk.class, Schulung.PERSISTENCE_UNIT);
        werkContainer.addNestedContainerProperty("firma.name");
    
        Table filterTable = new Table();
        // FilterTable filterTable = new FilterTable();
        filterTable.setContainerDataSource(werkContainer);
...}

I think it should work with addNestedContainerProperty, but i get an error "Invalid property name"

How could I solve this problem? Should I use another Container?

In the attachment you could find my two entities, they have a @ManyToOne relation