wx.canvasGetImageData 无法获取象素

https://developers.weixin.qq.com/miniprogram/dev/api/canvas/wx.canvasGetImageData.html

wx.canvasGetImageData({
  canvasId: 'canvasBox',
  x: 0,
  y: 0,
  width: 100,
  height: 100,
  success(res) {
    console.log(res.width) // 100
    console.log(res.height) // 100
    console.log(res.data instanceof Uint8ClampedArray) // true
    console.log(res.data.length) // 100 * 100 * 4
  },
   complete(res) { console.log( res ) }
})

{errMsg: “canvasGetImageData: fail canvas is empty”} api不生效