Hyperlink disappear after binding the text?

Hyperlink shows well in code below:

<TextBlock x:Name="textBlock"
                   Text = "Hello"><Hyperlink Name="Contacthyperlink" 
                     Foreground="Blue">
    world</Hyperlink></TextBlock>

But it just disappeared after change the TextBlock's text.

<TextBlock x:Name="textBlock"
                   Text = "{Binding Hello}"><Hyperlink Name="Contacthyperlink" 
                     Foreground="Blue">
    world</Hyperlink></TextBlock>

I have no idea about why does this happen, could anyone help?