HierachicalDataTemplate can't bind IEnumerable of an EntityRef Object

Hey there, 

simple task big problem:

 I have a hierachicalDataTemplate for an ORM Object , so foar so good all of the normal Properies like Name(String) or Note(string) a bound and even a simple Property in a EntityRef like Customer.Name gets bound but if I try to bind a IEnumerable of the Customer to a Combobox in the HierarichalDataTemplate   I get a binding error. The Debugger tells me there is no Property but there is and its perfectly filled with values.

If someone have a hint for me on that it would be cool, here are  my template and the interface definition 

<!-- TourPosition Template --><HierarchicalDataTemplate DataType="{x:Type Interfaces:ITourPosition}" x:Key="TourPositionItem"><Grid DataContext="{Binding }"><Grid.ColumnDefinitions><ColumnDefinition Width="Auto" /><ColumnDefinition Width="Auto" /><ColumnDefinition Width="Auto" /><ColumnDefinition Width="Auto" /><ColumnDefinition Width="Auto" /></Grid.ColumnDefinitions><Grid.RowDefinitions><RowDefinition Height="Auto" /></Grid.RowDefinitions><!-- Order --><TextBlock Grid.Row="0" Grid.Column="0" Text="{Binding Order}"                         
                       Background="Transparent" Margin="5"
				       Foreground="White" 
					   Width="50" 
					   HorizontalAlignment="Right"   VerticalAlignment="Center"  
				       FontSize="10" FontWeight="Normal"/><!-- Customer Name --><TextBlock Grid.Row="0" Grid.Column="1"  Text="{Binding Customer.Name}"                       
                       Background="Transparent" Margin="5"
				       Foreground="White" 
					   Width="150" 
					   HorizontalAlignment="Right"   VerticalAlignment="Center"  
				       FontSize="10" FontWeight="Normal"/><!-- Customer DeliveryAddress --><ComboBox Grid.Row="0" Grid.Column="2"  SelectedValuePath="Content"
                      BorderThickness="1" BorderBrush="{MyStyles:MyStyleRef ResourceKey=ForegroundBorderBrush}"
                      ItemsSource="{Binding Customer.DeliveryAddresses}"
                      ItemTemplate="{MyStyles:MyTemplateRef ResourceKey=AddressItem}"
                      SelectedItem="{Binding Address,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                      Style="{MyStyles:MyStyleRef ResourceKey=MyComboBoxStyle}"
                      Name="cbAddress"
                      Margin="10,5,5,5"
                      VerticalAlignment="Center"></ComboBox><!-- Note --><TextBox Grid.Row="0" Grid.Column="3"  Text="{Binding Note,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                     Style="{MyStyles:MyStyleRef ResourceKey=MyTextBoxStyle}"
                     TextWrapping="Wrap"
                     Background="Transparent" Margin="5"
				     SelectionBrush="{MyStyles:MyStyleRef ResourceKey=MySelectionBrush}"
			         BorderBrush="{MyStyles:MyStyleRef ResourceKey=ForegroundBorderBrush}"
				     Foreground="White" 
					 HorizontalContentAlignment="Stretch"  
					 MinWidth="200" MaxWidth="200"
					 HorizontalAlignment="Right"   VerticalAlignment="Center"  
				     FontSize="10" FontWeight="Normal"
					 CaretBrush="White"/><!-- Delete Button--><Button Grid.Row="0" Grid.Column="4" Content="-" Margin="10,5,5,5"
                    Style="{MyStyles:MyStyleRef ResourceKey=MyButtonStyle}"
                    Command="{Binding RelativeSource={RelativeSource FindAncestor,AncestorType={x:Type UserControl}},Path=DataContext.DeleteTourPositionCommand}"
                    CommandParameter="{Binding RelativeSource={RelativeSource FindAncestor,AncestorType={x:Type Grid}},Path=DataContext}"/></Grid></HierarchicalDataTemplate>

    public interface ITourPosition
    {
        Int64 ID { get; }
        ICustomer Customer { get; set; }
        string Note { get; set; }
        int Order { get; set; }
        ITour Tour { get; set; }
        IAddress Address { get; set; }
    }

    public interface ICustomer
    {
        IEnumerable<IEmail> CustomerEmail { get; }
        IEnumerable <IFax> CustomerFax { get; }
        IEnumerable <IPhone> CustomerPhone { get;  }
        IEnumerable <IAddress> DeliveryAddresses { get;  }
        IEnumerable <IDeliveryOrder > DeliveryOrder { get;  }
        decimal? Discount { get; set; }
        IEnumerable <IInvoice> Invoice { get;  }
        IEnumerable <IAddress> InvoiceAddress { get; }
        string Name { get; set; }
        string Note { get; set; }
        string Number { get; set; }
        Int64 ID { get; }
        IEnumerable <ICustomerPawn> Pawn { get;  }
        IEnumerable <ITour> Tour { get;  }
        IEmail MainEmail { get; }
        IFax MainFax { get; }
        IPhone MainPhone { get; }
        IAddress MainDeliveryAddress { get; }
        IAddress MainInvoiceAddress { get; }
    }

regards

Markus