title: Angular Async Pipe ngFor throwing an error
body: <p>I'm trying to populate a dropdown (reactive form) via a JSON file in a server. For some reason it keep showing the error </p>
  
  <blockquote>
    <p>TestComponent.html:14 ERROR Error: InvalidPipeArgument: '' for
    pipe 'AsyncPipe'</p>
  </blockquote>
  
  <p>here is how i set the value for my object in ngOninit()</p>
  
  <pre><code>  this.http.get(url).subscribe((data) =&gt; {
        for (const key in data) {
            this.NameData.push(data[key].Name)
        }
    })
  </code></pre>
  
  <p>Template code:</p>
  
  <pre><code>&lt;fieldset class="form-group"&gt;
      &lt;label for="nameType"&gt;Name Type:&lt;/label&gt;
      &lt;select class="form-control" id="nameType" formControlName="nameType"&gt;
          &lt;option *ngFor="let NType of NameData | async" [ngValue]="NType"&gt;{{NType}}&lt;/option&gt;
      &lt;/select&gt;
  &lt;/fieldset&gt;
  </code></pre>