Why does updating a Tree Grid with response metaData fail?

Sending metadata in a json response to a Tree Grid causes the control to silently fail in a loading state.

Chrome provides the following:

Code:

Uncaught TypeError: undefined is not a function
 appendChild
 appendChild
 Ext.cmd.derive.fillNode
 Ext.cmd.derive.onProxyLoad
 Ext.cmd.derive.triggerCallbacks
 Ext.cmd.derive.setCompleted
 Ext.cmd.derive.setSuccessful
 Ext.cmd.derive.process
 Ext.cmd.derive.processResponse
 (anonymous function)
 Ext.apply.callback
 Ext.cmd.derive.onComplete
 Ext.cmd.derive.onStateChange
 Ext.Function.j.bind

Here is the panel definition:

Code:

{
    xtype: 'treepanel',
    height: 250,
    width: 400,
    title: 'Tree Grid Panel',
    store: 'TreeStore',
    rootVisible: false,
    viewConfig: {


    },
    columns: [
        {
            xtype: 'treecolumn',
            dataIndex: 'key',
            text: 'Nodes',
            flex: 1
        },
        {
            xtype: 'gridcolumn',
            dataIndex: 'val',
            text: 'Value'
        }
    ]
}

This JSON response works as expected:

Code:

{
  "children": [
    {
      "avg": 3.4, 
      "children": [
        {
          "avg": 4.5, 
          "children": [], 
          "id": "slave-1", 
          "key": "alpha-v1", 
          "leaf": true, 
          "val": 1.5
        }
      ], 
      "id": "master-1", 
      "key": "alpha", 
      "leaf": false, 
      "val": 1.5
    }, 
    {
      "avg": 2.2, 
      "children": [
        {
          "avg": 1.1, 
          "children": [], 
          "id": "slave-2", 
          "key": "beta-v1", 
          "leaf": true, 
          "val": 2.25
        }
      ], 
      "id": "master-2", 
      "key": "beta", 
      "leaf": false, 
      "val": 2.25
    }
  ], 
  "offset": 0, 
  "total_count": 2
}

This final response fails:

Code:

{  "children": [
    {
      "avg": 3.4, 
      "children": [
        {
          "avg": 4.5, 
          "children": [], 
          "id": "slave-1", 
          "key": "alpha-v1", 
          "leaf": true, 
          "val": 1.5
        }
      ], 
      "id": "master-1", 
      "key": "alpha", 
      "leaf": false, 
      "val": 1.5
    }, 
    {
      "avg": 2.2, 
      "children": [
        {
          "avg": 1.1, 
          "children": [], 
          "id": "slave-2", 
          "key": "beta-v1", 
          "leaf": true, 
          "val": 2.25
        }
      ], 
      "id": "master-2", 
      "key": "beta", 
      "leaf": false, 
      "val": 2.25
    }
  ], 
  "metaData": {
    "columns": [
      {
        "dataIndex": "key", 
        "text": "Target", 
        "xtype": "treecolumn"
      }, 
      {
        "dataIndex": "val", 
        "text": "Last", 
        "xtype": "numbercolumn"
      }, 
      {
        "dataIndex": "avg", 
        "text": "Avg", 
        "xtype": "numbercolumn"
      }
    ], 
    "fields": [
      {
        "name": "key", 
        "type": "string"
      }, 
      {
        "text": "val", 
        "type": "float"
      }, 
      {
        "text": "avg", 
        "type": "float"
      }
    ]
  }, 
  "offset": 0, 
  "total_count": 2
}

Why does updating a Tree Grid with response metaData fail?