Dockpanel.dock="Top" not working

Question

I have a simple WPF application in which I'm trying to place a menu within a dockpanel. I have the following code:

<Grid>
    <DockPanel>
        <Menu Name="mnuMain" Height="30" DockPanel.Dock="Top" />
    </DockPanel>
</Grid>

The menu is appearing in the center of the dockpanel regardless of the setting of dockpanel.dock.

What am I overlooking?