Ribbon ComboBox WPF 4.5 c#

Question

Hello, I'm beginner here and would like your help.
 
The number select in dropdown don't appear in RibbonComboBox. Where I'm making mistakes? 

<RibbonWindow
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:Custom="http://schemas.microsoft.com/winfx/2006/xaml/presentation/ribbon" x:Class="RibbonComboBox.MainWindow"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Ribbon HorizontalAlignment="Left" VerticalAlignment="Top" Width="517" Height="113" SelectionChanged="Ribbon_SelectionChanged_1">
            <RibbonComboBox x:Name="teste" Label="ComboBox">
                <ComboBoxItem Content="2"/>
                <ComboBoxItem Content="3"/>
                <ComboBoxItem Content="4"/>
            </RibbonComboBox>
        </Ribbon>
</RibbonWindow>