When cursor hit an model it shows Uncaught TypeError: Caot read property 'emit' of undefined

I'm new to a-frame, so if the question is really easy please be patient with it. My question is that when a cursor hit an model it shows Uncaught TypeError: Caot read property 'emit' of undefined.Here is my code:

<a-scene>
  <a-assets>
    <img id="room" src="./pics/room.jpg">
    <a-asset-item id="crate-obj" src="/models/chair/Chair.obj"></a-asset-item>
    <a-asset-item id="crate-mtl" src="/models/chair/Chair.mtl"></a-asset-item>
  </a-assets>

  <a-entity position="0 1.8 4">
    <a-camera id="camera">
      <a-cursor id="cursor" color="#4CC3D9"></a-cursor>
    </a-camera>
  </a-entity>
  <a-obj-model src="#crate-obj" mtl="#crate-mtl"></a-obj-model>
  <a-sky src="#room"></a-sky>
</a-scene>