WindowsFormsHost doesn't resize panel

Hi I am trying to host a Panel in a WindowsformsHost control as follows:

    <DockPanel LastChildFill="true" Grid.Column="1" Grid.ColumnSpan="2">
       <WindowsFormsHost Name="FormsHost" Background="Aqua" Height="{Binding ElementName=RootWindow, Path=ActualHeight}" Width="{Binding ElementName=RootWindow, Path=ActualWidth}">
            <wf:Panel Dock="Fill" x:N开发者_如何学Pythoname="basePanel" AutoSize="True" />
        </WindowsFormsHost>
    </DockPanel>

for some reason the panel doesn't resize when the application does. Has anyone got any ideas?