Bootstrap-Vue Switch Style Checkbox: doesn't show custom control toggle

I'm reading through the documentation here for switch style checkboxes: https://bootstrap-vue.org/docs/components/form-checkbox

When I use the example HTML/JS in my own application, I can get the html to render but it doesn't show the actual switch toggle, like so:

https://jsfiddle.net/Lz5tcpqb/2/

<template>
  <div>
    <b-form-checkbox v-model="checked" name="check-button" switch>
      Switch Checkbox <b>(Checked: )</b>
    </b-form-checkbox>
  </div>
</template>

<script>
  export default {
    data() {
      return {
        checked: false
      }
    }
  }
</script>

Is there something I'm missing here?