I'm having a hard time binding a simple static string property to a text box.

Here's the class with the static property:

public class VersionManager
{
    private static string filterString;

    public static string FilterString
    {
        get { return filterString; }
        set { filterString = value; }
    }
}

In my xaml, I just want to bind this static property to a text box:

<TextBox>
    <TextBox.Text>
        <Binding Source="{x:Static local:VersionManager.FilterString}"/>
    </TextBox.Text>
</TextBox>

Everything compiles, but at run time, I get the following exception:

Cannot convert the value in attribute 'Source' to object of type 'System.Windows.Markup.StaticExtension'. Error at object 'System.Windows.Data.Binding' in markup file 'BurnDisk;component/selectversionpagefunction.xaml' Line 57 Position 29.

Any idea what I'm doing wrong?