KEYPAD_EVENT event是个nil value

function HeroScene:onEnter()
    if device.platform == "android" then
        self:performWithDelay(function()
            local layer = display.newLayer()
            layer:addNodeEventListener(cc.KEYPAD_EVENT, function(event)
                if event.key == "back" then 
                    return app:enterScene("MainScene") 
                end
            end)
            self:addChild(layer)
            layer:setKeypadEnabled(true)
        end, 0.2)
    end
end

提示event 是个nil value,223可以用,升级到master后出现错误,点解?

请查看 http://www.cocoachina.com/bbs/read.php?tid=213391#999257