组合框索引获取[object Object]而不是extjs中的数字

I have and extjs task running in Ext.TaskManager which reloads my ext store every 5 seconds. Each time the task is called I update currentIndex variable that holds the currently selected index(value) in my combobox. The probelm i get is that in some circumstances instead of getting number currentIndex gets value [object Object]. I really have no idea why this happens. Here is my sample code:I have and extjs task running in Ext.TaskManage