DataTrigger in XAML throwing an error

Hello.
I'm trying to use DataTrigger from sample http://blog.xamarin.com/triggers-in-xamarin.forms/ but it throws error.

<Entry x:Name="emailAddress" Text="" Placeholder="email address"/>
                        <Button Text="Send">
                            <Button.Triggers>
                                <DataTrigger TargetType="Button" Binding="{Binding Source={x:Reference emailAddress}, Path=Text.Length}" Value="0">
                                    <Setter Property="IsEnabled" Value="False" />
                                </DataTrigger>
                            </Button.Triggers>
                        </Button>

It's bug?

Forms version : 1.3.5.6337 (1.4pre2 does not work too).