title: vue.js .sync can't read property of undefined
body: <p>My child component looks like this:</p>
  
  <pre><code>&lt;div&gt;
                  &lt;label&gt;Label&lt;/label&gt;
                  &lt;VTextField
                    :value="addOnLabel"
                    @input="$emit('update:addOnLabel', $event.target.value)"
                    solo
                    outline
                    reverse
                    type="text"
                  /&gt;
                &lt;/div&gt;
  
  props: {
      addOnLabel: {},
    },
  </code></pre>
  
  <p>My parent looks like this:</p>
  
  <pre><code>&lt;MultiplierDropDown :addOnLabel.sync="addOnLabel"/&gt;
  </code></pre>
  
  <p>Problem: as soon as I start typing something in the input field, it says <code>Error in v-on handler: "TypeError: Cannot read property 'value' of undefined"</code></p>