"canvasToTempFilePath: fail canvas is empty"？

<ec-canvas id="mychart" canvas-id="myCanvas" ec="{{ ec }}" force-use-old-canvas="true"></ec-canvas>

canvasId是有的，t第二个参数this，也有，wx.canvasToTempFilePath（）调用就报错，报错信息："canvasToTempFilePath: fail canvas is empty"，求指点

setTimeout(() => {

      console.log('this=====',this)

wx.canvasToTempFilePath({

        canvasId: 'myCanvas',

        width:'200',

        height:'300',

        success: (res) => {

          console.log("res",res)

        },

        fail: (err) => {

          console.log("err",err)

        },

      }, this)

    }, 2000)