title: TypeError: this._parentSubscription.unsubscribe is not a function
body: <p>What is causing this TypeError when the observable encounters an error? The error led me to believe that for some reason <code>fbLogin.unsubscribe();</code> was the issue, but even with both of those lines removed, the error persists. I am using rxjs@^5.5.2.</p>
  
  <pre><code>ERROR TypeError: this._parentSubscription.unsubscribe is not a function
      at Subscriber._unsubscribeParentSubscription (Subscriber.js:110)
      at Subscriber.error (Subscriber.js:75)
      at auth.ts:54
      at e.b (auth.esm.js:17)
      at Fb (auth.esm.js:20)
      at Bb (auth.esm.js:20)
      at A.g.Xb (auth.esm.js:19)
      at kb (auth.esm.js:13)
      at t.invoke (polyfills.js:3)
      at Object.onInvoke (core.js:4760)
  
  
  //login.ts
  loginWithFacebook(): void{
          let fbLogin = this.authData.loginWithFacebook().subscribe(() =&gt; {
              fbLogin.unsubscribe();
              this.navCtrl.setRoot('HomePage');
          }, error =&gt; {
              fbLogin.unsubscribe();
              console.log(error);
          });
      }
  
  
  //auth.ts
  loginWithFacebook(): Observable&lt;any&gt; {
          return Observable.create(observer =&gt; {
              if (this.platform.is('cordova')) {
                  return this.fb.login(['email', 'public_profile']).then(res =&gt; {
                      const facebookCredential = firebase.auth.FacebookAuthProvider.credential(res.authResponse.accessToken);
                      this.afAuth.auth.signInWithCredential(facebookCredential).then(()=&gt; {
                          observer.next();
                      }).catch(error =&gt; {
                          console.log(error);
                          observer.error(error);
                      });
                  });
              } else {
                  return this.afAuth.auth.signInWithPopup(new firebase.auth.FacebookAuthProvider()).then(()=&gt; {
                      observer.next();
                  }).catch(error =&gt; {
                      console.log(error);
                      observer.error(error); //this is auth.ts:54
                  });
              }
          });
      }
  </code></pre>