Javascript callWS issues

Hello guys, I’m creating my own frontend component, I’m trying to callWS (Follow this docs) to get mqtt message to display, but I always got null at the first response, then when I send another message to topic, I got

Received event for unknown subscription 41. Unsubscribing. 

This is my code,

hass.callWS({
  type: 'subscribe_trigger',
  trigger: {
    platform: 'mqtt',
    topic: 'mytopic'
  }
}).then(async res => {
  console.log(res); // Return null
})

How I can fix it?
Thank you