Adding "Authorization" header in get request

I trying

  let headers = new Headers({ 'Authorization': 'Bearer ' + window.localStorage.getItem('token') });
    let options = new RequestOptions({ headers: headers });
    return this.http.get(this.api+'causes' , options).map( res => res.json());

but return to me the following error
ERROR Error: Cannot find a differ supporting object '[object Object]' of type 'object'. NgFor only supports binding to Iterables such as Arrays. ??
Any idea ?