bootstrap-vue: Radio button throws "Uncaught TypeError: this.checked.includes is not a function"

I tried (https://codepen.io/WoJWoJ/pen/YQLqjG) the simplest radio button (includes per the documentation):

<div id="root">
  <b-form-checkbox v-model="selected">something</b-form-checkbox>
  <div>
    The checkbox is selected: {{selected}}
  </div>
</div>

new Vue ({
  el: "#root",
  data: {
    selected: true
  }
})

The output is

something
The checkbox is selected: {{selected}}

and the Chrome DevTools show

vue.min.js:6 Uncaught TypeError: this.checked.includes is not a function
    at a.isChecked (form-checkbox.vue:43)
    at no.get (vue.min.js:7)
    at no.evaluate (vue.min.js:7)
    at a.isChecked (vue.min.js:6)
    at a.render (form-checkbox.vue:24)
    at a.n._render (vue.min.js:6)
    at a.<anonymous> (vue.min.js:6)
    at no.get (vue.min.js:7)
    at new no (vue.min.js:7)
    at a.e._mount (vue.min.js:6)