JSON not parsing into model

Hi

I am sure I am missing something here but I am parsing some json and ONE field never gets parsed into the record.

the field "caTypeDescription" is always empty as can be seen from an example of the data below

Code:

data: Object
   announcementId: 8121
   caType: "DVOP"
   caTypeDescription: ""
   createdBy: ""
   dateCreated: null
   financialYearEnd: ""
   id: "iActs.model.Announcements-2"
   issuerDetails: Object
   issuerName: "TESTENTITY"
   mandVolChoiceFlag: "M"
   securitiesId: 0
   updateTypeFlag: "NEWM"

My model is as so

Code:

Ext.define('iActs.model.Announcements', {
            extend : 'Ext.data.Model',
            id : 'announcementId',
            fields : [ {
                        name : 'announcementId',
                        type : 'int'
                    }, {
                        name : 'securitiesId',
                        type : 'int'
                    }, {
                        name : 'issuerName',
                        type : 'string',
                        mapping : 'issuerDetails.name'
                    }, {
                        name : 'caType',
                        type : 'string',
                        mapping : 'caType.caType'
                    }, {
                        name : 'caTypeDescription',
                        type : 'string',
                        mapping : 'caType.description'
                    }, {
                        name : 'mandVolChoiceFlag',
                        type : 'string'
                    }, {
                        name : 'updateTypeFlag',
                        type : 'string'
                    }, {
                        name : 'financialYearEnd',
                        type : 'string'
                    }, {
                        name : 'dateCreated',
                        type : 'date'
                    }, {
                        name : 'createdBy',
                        type : 'string'
                    }]
        });

and the JSON

Code:

{"root": [
    {
        "announcementId": 8121,
        "issuerDetails": {
            "entity": "XX",
            "name": "TESTENTITY",
            "issuerId": "ISSUER01"
        },
        "caType": {
            "caType": "DVOP",
            "description": "Dividend Option"
         },
        "mandVolChoiceFlag": "M",
        "updateTypeFlag": "NEWM",
        "financialYearEnd": null,
        "dateCreated": null,
        "createdBy": null
     }
]}

As you can see caType.caType is parsed, but caType.description on this and every other record I have tried is blank.

Either it is a bug or I am missing something so any help would be appreciated as this is driving me up the wall!

Thanks in advance

JSON not parsing into model