SplitChild Mixin Exception

gistfile1.js

// ==========================================================================
// Project:   Resumes - mainPage
// Copyright: ©2011 My Company, Inc.
// ==========================================================================
/*globals Resumes */

// This page describes the main user interface for your application.  
Resumes.mainPage = SC.Page.design({
  mainPane: SC.MainPane.design({
    childViews: 'splitView'.w(),
    
    splitView: SC.SplitView.design({
      childViews: 'leftPanel rightPanel'.w(),
      
      leftPanel: SC.View.design(SC.SplitChild, {
        minimumSize: 200
      }),

      rightPanel: SC.View.design(SC.SplitChild, {
        autoResizeStyle: SC.RESIZE_AUTOMATIC
      })
    })
  })
});

Uncaught exception: SC.Object.extend expects a non-null value. Did you forget to 'sc_require' something? Or were you passing a Protocol to extend() as if it were a mixin?

Exception is being thrown with the SC.SplitChild mixin. This is straight from the edge docs. No further information is given on how to resolve...