Can't create custom more info card


I’m trying to create a new more info card but I’m getting an error extending Polymer.Element, If I past the script on the browser console, it works, but when I’m loading the component I get Uncaught TypeError: Class extends value undefined is not a constructor or null

My code:

<dom-module id='more-info-plant'>


  class MoreInfoPlant extends Polymer.Element {
    static get is() { return 'more-info-plant'; }

    static get properties() {
      return {
        // Home Assistant object
        hass: Object,
        // inDialog is true if shown as more-info-card
        inDialog: {
          type: Boolean,
          value: true,
        // includes state, config and more information of the entity
        stateObj: Object,

    _toStr(obj) {
      return JSON.stringify(obj, null, 2);

  customElements.define(, MoreInfoPlant);