event.originalEvent undefined in overridden focus event

I need to override the autocomplete focus event with a custom action and do something based on whether the user was selecting from the list using the keyboard and not the mouse. To do that I'm checking the originalEvent.type in the event object which is supposed to contain the type of action performed (keydown, keyup, mouseenter, etc).

However, the originalEvent object seems to be undefined. It is working just fine in the actual focus event in the autocomplete code, but not when I override that event inside of the autocomplete object.

Please see my code below.

$( "#tags" ).autocomplete({
    source: availableTags,
    focus: function(event, ui) {
        //Check whether focus was triggered by a mouse or keyboard event
        if ( /^key/.test(event.originalEvent.type) ) {
            //Do something here
        }                   

        return false;
    }
});