Enabling a button in WPF depending on ListBox.SelectedIndex

<Button Name="btn1" >click me
    <Button.Style>
        <Style>
            <Style.Triggers>
                <DataTrigger
                     Binding ="{Binding ElementName=list1, Path=SelectedIndex}"
                     Value="-1">
                    <Setter Property="Button.IsEnabled" Value="false"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Button.Style>
</Button>