C# 按钮启用绑定到组合框

我需要一个按钮禁用，直到3个文本框是空的，没有选择一个组合框做。我编写了以下XAML代码：

<Button x:Name="previewBtn" Content="Anteprima" HorizontalAlignment="Left" Margin="164,33,0,0" VerticalAlignment="Top" Width="75" Height="19" Click="previewBtn_Click_1">
        <Button.Style>
            <Style TargetType="Button">
                <Style.Triggers>
                    <DataTrigger Binding="{Binding Text.Length, ElementName=titleBox}" Value="0">
                        <Setter Property="IsEnabled" Value="False" />
                    </DataTrigger>
                    <DataTrigger Binding="{Binding Text.Length, ElementName=StEpLabel}" Value="0">
                        <Setter Property="IsEnabled" Value="False" />
                    </DataTrigger>
                    <DataTrigger Binding="{Binding Text.Length, ElementName=seasonBox}" Value="0">
                        <Setter Property="IsEnabled" Value="False" />
                    </DataTrigger>
                    <DataTrigger Binding="{Binding SelectedIndex, ElementName=cmb}" Value="0">
                        <Setter Property="IsEnabled" Value="False" />
                    </DataTrigger>
                </Style.Triggers>
            </Style>
        </Button.Style>
    </Button>

绑定到textboes可以工作，但组合框不能。我应该写什么？谢谢

<DataTrigger Binding="{Binding SelectedIndex, ElementName=cmb}" Value="-1">
         <Setter Property="IsEnabled" Value="False" />
  </DataTrigger>

未选择任何项目时，SelectedIndex将为-1