vue.js numeric checkbox value

I need a checkbox binded to a number field, but it always have string type or boolean

<input v-if="item.price.TypeId==1" type="checkbox" class="form-control form-control-sm" v-model.number="item.qty" true-value="1" false-value="0" v-on:change="calcSum(item)">

after checking, item.qty have string value, v-model.number not work, how to make numeric?