Why doesn't Vue render the object's value?

Question

Vue.js

There is a component

...
<button class="persons-counter-m" @click.prevent="deliveryDecriment()"></button>
<span>{{ formModel.persons }}</span>
<button class="persons-counter-p" @click.prevent="formModel.persons++"></button>
...

...
  data() {
    return {
      cartConfig,
      formModel: {
        ...
        persons: 1
      },
   }
...

And now formModel.persons does not want to be rendered in any way and displays NaN on manipulations like incrementing and decrementing. Why does this happen?