title: InvalidPipeArgument: '[object Object]' for pipe 'AsyncPipe' Angular 6 and FIrebase
body: <p>**Error: InvalidPipeArgument: '[object Object]' for pipe 'AsyncPipe'.
  **</p>
  <p>I am trying to do unsubscribe the observable</p>
  <p>Angular-cli = 6.0.3</p>
  <blockquote>
  <p>HTML home.component.html</p>
  </blockquote>
  <pre><code>&lt;div class=&quot;starter-template&quot;&gt;
    &lt;h1&gt;Firebase Data&lt;/h1&gt;
    &lt;ul&gt;
      &lt;li *ngFor=&quot;let course of courses$ | async&quot;&gt;
        {{ course.$value }}
      &lt;/li&gt;
    &lt;/ul&gt;
  &lt;/div&gt;
  </code></pre>
  <blockquote>
  <p>Typescript  home.component.ts</p>
  </blockquote>
  <pre><code>export class HomeComponent {
    courses$;
    constructor(db: AngularFireDatabase) {
      this.courses$ = db.list('/course');
      }
  }
  </code></pre>