md-autocomplete "required" validation not applied to parent form
<p>Following the provided <a href="https://material.angularjs.org/latest/api/directive/mdAutocomplete" rel="nofollow">documentation</a> in angular material, I have setup a working autocomplete directive. </p>

<p>The required validation is being shown in the nested error message "div", but there is no effect on the parent form.
The parent form seems to be valid even for empty value in autocomplete.</p>

<pre><code>&lt;form ng-submit="ngSubmit()" layout="column" class="md-padding"
    name="itemsForm" flex&gt;

     &lt;md-input-container&gt;
       &lt;label&gt;Code *&lt;/label&gt;
       &lt;input type="text" name="code" required ng-model="ngModel.code"/&gt;
     &lt;/md-input-container&gt;

      &lt;md-autocomplete flex
                   required
                   md-input-name="author"
                   md-search-text="searchText"
                   md-search-text-change="ngModel.author = (searchText)"
                   md-selected-item-change="ngModel.author = item"
                   md-items="item in filterAutocomplete(searchText)"
                   md-item-text="item"
                   md-floating-label="Author *"&gt;
         &lt;md-item-template&gt;
           &lt;span md-highlight-text="ctrl.searchText"&gt;{{item}}&lt;/span&gt;
         &lt;/md-item-template&gt;
         &lt;div ng-messages="autocompleteForm.autocomplete.$error"&gt;
           &lt;div ng-message="required"&gt;This field is required&lt;/div&gt;
         &lt;/div&gt;
     &lt;/md-autocomplete&gt;
    &lt;md-button class="md-primary" type="submit"
             ng-disabled="itemsForm.$invalid"
             ng-class="{'md-raised': itemsForm}"&gt;
    Save
  &lt;/md-button&gt;
&lt;/form&gt;
</code></pre>

<p>How can I achieve the similar behaviour to normal required input fields in side a form, when using md-autocomplete?</p>