For Extjs combobox ,UP/DOWN arrow keys do not work in IE8

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ExtJS

Ext.view.BoundListKeyNav.override({
highlightAt:function (index) {
var boundList = this.boundList;
var litem = boundList.all.item(index);
if (litem) {
litem = litem.dom;
boundList.highlightItem(litem);
boundList.getTargetEl().scrollChildIntoView(litem, true);
combo.setValue(boundList.getNode(index).textContent);
}
}
});

Using this code,to show the content on the combobox textfield in extjs when UP/DOWN arrow keys are pressed.But this does not work in IE8.When i type in the combobox,it shows me the search results and as soon as i press the DOWN/UP arrow key,the search results vanish away and the textfield remains blank.What could be the reason for this?......Thanks