Vue property not valid with underscore

I was doing some tests and I noticed that my properties are not valid when I used underscore.

Example:

new Vue({
el : "#form",

data: {
    errors: [],
    _username: '',
    _password: ''
});

on html file:

<input class="uk-input" type="text" v-model="_username" >
<input class="uk-input" type="password" v-model="_password">

With the code above the app won't render. If I remove the underscore it will work, does someone knows why this happens?