Strange "INavigatorContent" error compiling in 4.0

I've recently decided to try an upgrade to 4.0.  The only error I still can't work out is this one:

"The children of Halo navigators must implement INavigatorContent"

I seem to be getting it on all my ViewStacks that have validators.

     <mx:ViewStack xmlns:mx="http://www.adobe.com/2006/mxml">
     <mx:NumberValidator id="systolicValidator"
          source="{systolic}"
          required="true"
             property="text"
          minValue="10"
          maxValue="300"
          domain="int"/>
     <mx:NumberValidator id="diastolicValidator"
          source="{diastolic}"
          required="true"
          property="text"
          minValue="10"
          maxValue="200"
          domain="int"/>
 
        <mx:TextInput id="systolic"/>
        <mx:TextInput id="diastolic"/>
           ...
      </ViewStack>

My compiler is set to "flex 3 compatibility mode" and my theme is set to Halo (default).

Any ideas what I might be missing?