TypeError: is not a function in TypeScript class

I'm getting the following error in my TypeScript class. I am trying to call a helper function passing the token.

Error:

post error: TypeError: this.storeToken is not a function(…)
​

Class:

/**
* Authentication Service:
*
* Contains the http request logic to authenticate the
* user.
*/
import { Injectable } from '@angular/core';
import { Http, Response, Headers, RequestOptions } from '@angular/http';

import 'rxjs/Rx';
import { Observable } from 'rxjs/Observable';

import { AuthToken } from './auth-token.service';

import { User } from '../../shared/models/user.model';

@Injectable()
export class Authenticate {

constructor(
private http: Http,
private authToken: AuthToken
) {}

post(user: User): Observable<any> {
let url = 'http://localhost:4000/';
let body = JSON.stringify(user);
let headers = new Headers({ 'content-type': 'application/json' });
let options = new RequestOptions({ headers: headers });

return this.http.post(url + 'login', body, options)
.map(this.handleData)
.catch(this.handleError);
}

private storeToken(token: string) {
this.authToken.setToken(token);
}

private handleData(res: Response) {
let body = res.json();
this.storeToken(body.token);
return body.fields || {};
}

private handleError(error: any) {
console.error('post error: ', error);
return Observable.throw(error.statusText);
}
}

Continue reading...