:class не работает динамически

new Vue({ 
  el: "#app", 
      data(){ 
        return{ 
          form: { 
            storeData: 0 
          } 
        } 
      }, 
      methods:{ 
        clickStoreData: function(index){ 
        	this.form.storeData = index-1 
        } 
      }, 
})

    fieldset .wrap_radio{ 
        margin: 5px 0 5px 0; 
        height: 30px; 
        width: 200px; 
        font: 400 15px/30px arial; 
        color: #000; 
        text-align:left; 
        display: block; 
    } 
    fieldset .wrap_radio .radio{ 
        border: 3px solid #852005; 
        margin: 7px 1px 5px 1px; 
        border-radius:12px; 
        height: 15px; 
        width: 30px; 
        background-color: #ff363d; 
        /*float: right;*/ 
        cursor: pointer; 
        box-sizing: border-box; 
    } 
    fieldset .wrap_radio .radio.active{ 
        border: 3px solid #024406; 
        background-color: #019163; 
    }

<script src="https://unpkg.com/vue@2.5.16/dist/vue.js"></script> 
    <fieldset id="app"> 
        <div class="wrap_radio">Store Data? Yes/No 
            <div  
                 v-for="index in 2" 
                 @click="clickStoreData(index)" 
                 class="radio" 
                 :class="{'active': form.storeData == index-1}" 
                 > 
            </div> 
        </div> 
    </fieldset>