Binding to a custom control

I'm having difficulty binding my ViewModel to a custom control. On the same View, binding to a regular control works but the custom control is blank.

Here's the custom control:

public class CustomEntry : StackLayout
{
    protected Entry entry;

    public CustomEntry ()
    {
        entry = new Entry ();

        Children.Add (entry);
    }

    public static readonly BindableProperty TextProperty=BindableProperty.Create<CustomEntry, string>( p => p.Text, "" );

    public String Text {
        get { return entry.Text; }
        set { entry.Text = value; }
    }
}

And the view:

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    x:Class="TestMVVM.TestPage"
    xmlns:mv="clr-namespace:TestMVVM"
    Padding="10, 20">
    <ContentPage.BindingContext>
        <mv:TestViewModel />
    </ContentPage.BindingContext>
    <StackLayout>
        <Entry Text="{Binding TestValue}" />
        <mv:CustomEntry Text="{Binding TestValue}" />
    </StackLayout>
</ContentPage>

And the viewmodel:

public class TestViewModel : INotifyPropertyChanged
{
    double testValue;

    public event PropertyChangedEventHandler PropertyChanged;

    public TestViewModel ()
    {
        var random = new Random ();
        testValue = random.NextDouble ();
    }

    public double TestValue 
    {
        set {
            if (testValue != value) {
                testValue = value;
                OnPropertyChanged ("TestValue");
            }
        }
        get {
            return testValue;
        }
    }

    protected virtual void OnPropertyChanged (string propertyName) {
        if (PropertyChanged != null) {
            PropertyChanged (this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

I put the full code up on GitHub: [https://github.com/andybarilla/BindableCustomTest]