Uncaught TypeError: undefined is not a function while Store loading

I have a store

Code:

Ext.define('MyApp.store.Users', {
    extend: 'Ext.data.Store',
    id: 'UsersStore',
    requires: [
        'MyApp.model.Users'
    ],
    config: {
        model: 'MyApp.model.Users',
        sorters: 'lastName',
        proxy: {
            type: 'ajax',
            url: '/users.json',
            extraParams: {
            },
            reader: {
                type: 'json',
                rootProperty: 'items'
            }
        },
        autoLoad: false
    }
});

I'm trying to create and load it

Code:

Ext.create('MyApp.store.Users');
var a = Ext.getStore('UsersStore').getProxy().load({
});

And in load() I receiving message:

Uncaught TypeError: undefined is not a function

I don't know what a reason of this problem and how I can to solve it.