Angular 5 Observable and ngModel Error: Cannot read property 'title' of undefined

I'm building a form using [(ngModel)]="article.title" for one of the input fields. I know I can use the safe navigation operator in string interpolation but I want to use two way data binding.

The console is showing the following error:

Cannot read property 'title' of undefined
    at Object.View_EditPostComponent_0._co [as updateDirectives] (EditPostComponent.html:11)
    at Object.debugUpdateDirectives [as updateDirectives] (core.js:14689)
    at checkAndUpdateView (core.js:13836)
    at callViewAction (core.js:14187)
    at execComponentViewsAction (core.js:14119)
    at checkAndUpdateView (core.js:13842)
    at callViewAction (core.js:14187)
    at execEmbeddedViewsAction (core.js:14145)
    at checkAndUpdateView (core.js:13837)
    at callViewAction (core.js:14187)

The Template is:

<input ... [ngModel)]="article.title" ... />

The Component:

export class EditPostComponent implements OnInit {

[...]

createPostForm: FormGroup;
urlParam: any;
article: any;

[...]

ngOnInit() {
  this.urlParam = this.activatedRoute.snapshot.params;

  this.blogService.getArticle(this.urlParam.id)
    .subscribe(data => {this.article = data});
}

Using string interpolation with the Elvis operator works fine and doesn't throw any error, but ngModel is giving the error.

Could somebody tell me why is that and suggest a solution? Thanks