Running unit tests from ant: flashplayer window remains open

I'm trying to run flex unit tests from ant but I can't get the flashplayer window to close afterwards. What am I doing wrong?

<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
                xmlns:s="library://ns.adobe.com/flex/spark"
                xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600"
                xmlns:adobe="http://www.adobe.com/2009/flexUnitUIRunner"
                creationComplete="onCreationComplete()">
     <fx:Script>
         <![CDATA[ 
             import sampleSuite.WidgetTest;

            //import org.flexunit.listeners.UIListener;

            import org.flexunit.runner.FlexUnitCore;
             //import org.flexunit.listeners.CIListener;                       
             private var core:FlexUnitCore;

            private function onCreationComplete():void
             {
                 //testRunner.runWithFlexUnit4Runner(currentRunTestSuite(), "deleteme");
                 core = new FlexUnitCore();              
                 core.run(WidgetTest);
             }        

        ]]>
     </fx:Script>
     <fx:Declarations>

        <!-- Place non-visual elements (e.g., services, value objects) here -->
    </fx:Declarations>
   
</s:Application>

and my ant script looks like this

<target name="test" depends="build-test">  <!--  -->
         <taskdef resource="flexUnitTasks.tasks" classpath="./flexunit/flexUnitTasks-4.0.0.jar"/>
         <flexunit swf="./bin/${tester}" verbose="true"  localTrusted="true" timeout="15000" headless="true" toDir="./ASCoverage"  haltonfailure="true"/>
</target>