关于jquery combobox

在网上找到一个关于 jquery combobox的代码,
里面用一段代码是这样的:
.bind("autocompleteopen", function(event, ui){
                        //find the currently selected item and highlight it in the list
                        var opt = select.children(":selected")[0];
                        input.data("autocomplete").widget().children('li').each(function() {        
                            var $li = $(this);
                            if ($li.data("item.autocomplete").option == opt) {
                                input.data("autocomplete").menu.activate(event,$li);
                                return false;
                            }
                        });
                    });
但是我用firebug调试的时候总是告诉我, input.data("autocomplete").menu.activate(event,$li);这一句里面的activate不时一个function.
小弟初学js,请教一下各位大大,这是一个什么错误呢,谢谢