I'm getting the following error while calling sync() on a buffered store:

Uncaught TypeError: Object [object Object] has no method 'filterBy' ext-all-debug.js:65308
Ext.define.getNewRecords ext-all-debug.js:65308
Ext.define.sync ext-all-debug.js:61855
Ext.define.fireHandler ext-all-debug.js:43790
Ext.define.onClick ext-all-debug.js:43780
Ext.apply.createListenerWrap.wrap ext-all-debug.js:10303

var store = Ext.create('Ext.data.Store', {
id: 'store',
buffered: true,
pageSize: 5000,
model: 'Employee',

proxy: {
type: 'ajax',
method : 'GET',
api: {
read: 'getEmployees.xml',
update: 'setEmployees.html'
reader: {
root: 'employees',
record: 'employee',
type: 'xml'
writer: {
documentRoot: 'employees',
record: 'employee',
type: 'xml',
writeAllFields: false

After some investigation on my own I noticed that a buffered store uses Ext.util.LruCache which lacks the method filterBy that Ext.util.AbstractMixedCollection has, which seems to be used on a non-buffered store.
Can someone confirm if this is a bug or if I'm doing this the wrong way?