I think this is also a bug. This bug happens when you extend from a model that has one or more auto field. Below is my fix for this bug. Hope that will help.
Code:
Ext.define('FixedDataTypes', {
    override: 'Ext.data.Types',
    AUTO: {
        convert: function(v){
            return v;
        },
        sortType: Ext.data.SortTypes.none,
        type: 'auto'
    }
});
BTW, I have posted this bug on the Bugs branch. See it here http://www.sencha.com/forum/showthre...th-auto-fields