Changing Tab ImageSource using DataTrigger

Hi,

I am trying to change the tab ImageSource to an hourglass if my ViewModel is in a running state. I thought that I should be able to do something like in my view.xaml:

<dock:DocumentWindow x:Class="Test.View"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:shared="http://schemas.actiprosoftware.com/winfx/xaml/shared"
xmlns:dock="http://schemas.actiprosoftware.com/winfx/xaml/docking"
xmlns:propgrid="http://schemas.actiprosoftware.com/winfx/xaml/propgrid"
xmlns:l="clr-namespace:Test.ViewModels"
Title="{Binding DisplayName, Mode=TwoWay}">

<dock:DocumentWindow.Resources>
<Style TargetType="{x:Type dock:DocumentWindow}" >
<Style.Triggers>
<DataTrigger Binding="{Binding ReportRunningState, Mode=OneWay}"
Value="{x:Static l:ReportState.Running}"
<Setter Property="ImageSource" Value="/Resources/Images/hourglass16.png"/>
</DataTrigger>
<DataTrigger Binding="{Binding ReportRunningState, Mode=OneWay}"
Value="{x:Static l:ReportState.Empty}">
<Setter Property="ImageSource" Value="{x:Null}" />
</DataTrigger>
</Style.Triggers>
</Style>
</dock:DocumentWindow.Resources>

Doing this, the view appears but the tab headers have disappeared. Any ideas what is wrong or how i might acomplish this?

cheers