赋值后,用户控件中的依赖项属性为空

我有一个用户控件,有一个框架作为依赖属性

public partial class NavigationBar : UserControl
{
    public NavigationBar()
    {
        this.InitializeComponent();
    }

    public Frame NavigationFrame
    {
        get { return (Frame)GetValue(NavigationFrameProperty); }
        set { SetValue(NavigationFrameProperty, value); }
    }

    // Using a DependencyProperty as the backing store for NavigationFrame.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty NavigationFrameProperty =
        DependencyProperty.Register("NavigationFrame", typeof(Frame), typeof(NavigationBar), new UIPropertyMetadata());
}

<userControls:NavigationBar NavigationFrame="{Binding ElementName=masterPage, Path=frameNavigations}" />

 <Frame x:Name="frameNavigations"/>

提前谢谢