main.lua:25:尝试调用方法'setanchorPoint'(一个空值)堆栈追溯:main.lua 25:在主块中maiin

我遇到了一个错误: main.lua:25:尝试调用方法'setanchorPoint'(一个空值)堆栈追溯:main.lua 25:在主块中maiin. 请告诉我如何使用anchorPoint。如何解决这个问题

--常量
_H = display.contentHeight;
_W = display.contentWidth;
mRand = math.random;
o = 0;
time_remain = 10;
time_up = false;
total_orbs = 20;
ready = false;
--准备要播放或访问的声音
local soundtrack = audio.loadStream("media/soundtrack.caf");
local pop_sound = audio.loadSound("media/pop.caf");
local win_sound = audio.loadSound("media/win.caf");
local fail_sound = audio.loadSound("media/fail.caf");

local display_txt = display.newText("等待", 0, 0, native.systemFont, 16*2);
display_txt.xScale = .5; display_txt.yScale = .5;
display_txt:setanchorPoint(display.BottomLeftanchorPoint);
display_txt.x = 20; display_txt.y = _H-20;