Cannot read property 'indexOf' of undefined

I don't understand why I'm getting this error, all I'm trying to do is associate a generic store with a combobox to display the values from the store and I keep getting this error for some reason.

comboBox:

Code:

{
                            xtype: 'combobox',
                            anchor: '100%',
                            itemId: 'testComboBox',
                            maxWidth: 300,
                            width: 1130,
                            fieldLabel: 'Type',
                            labelWidth: 50,
                            displayField: 'label',
                            store: 'TestStore',
                            valueField: 'label'
                        }

store:

Code:

Ext.define('MyApp.store.TestStore', {
    extend: 'Ext.data.Store',


    requires: [
        'MyApp.model.TestModel'
    ],


    constructor: function(cfg) {
        var me = this;
        cfg = cfg || {};
        me.callParent([Ext.apply({
            autoLoad: true,
            model: 'MyApp.model.TestModel',
            storeId: 'TestStore',
            data: [
                {
                    catid: '1',
                    label: 'issue 1',
                    type: 'incident'
                },
                {
                    catid: '2',
                    label: 'issue 2',
                    type: 'incident'
                },
                {
                    catid: '3',
                    label: 'issue 3',
                    type: 'incident'
                },
                {
                    catid: '4',
                    label: 'issue 4',
                    type: 'incident'
                }
            ]
        }, cfg)]);
    }
});

model:

Code:

Ext.define('MyApp.model.TestModel', {
    extend: 'Ext.data.Model',


    requires: [
        'Ext.data.Field'
    ],


    idProperty: 'catid',


    fields: [
        {
            name: 'catid'
        },
        {
            name: 'label'
        },
        {
            name: 'type'
        }
    ]
});

the combo box displays the values when i click on the drop down button, but it gets stuck with a loading message and I get the error mentioned above, could someone tell me what the problem is here please? am I missing something?