I'm having trouble getting a vbox panel inside of an hbox panel to show up. The vbox panel always has 0 height regardless of its children. I've reduced it to a simple example that illustrates the issue

Code:
items: [{
   layout: 'hbox',
   items: [{
      layout: 'vbox',
      items: [{
            html: 'left-top'
         },{
            html: 'left-bottom'
         }]
      },{
         html: 'center'
      },{
         html: 'right'
   }]
}]
The center and right panels are visible, but the left panel has 0 height. Giving it a fixed height works, but is not an option since the content is dynamic.

Any suggestions would be greatly appreciated.
Thanks.