错误：422(不可处理的实体). Angular4

无法加载资源：服务器响应状态为422(不可处理的实体)

这是发布到api的服务：

@Injectable()
export class ApiService {
    constructor(private http: Http) { }
    login(username: string,password: string): Observable<Response>{
        const url = 'http://localhost:8000/login';
        const json = JSON.stringify({username: username,password: password});
        const headers: Headers = new Headers();
        headers.append('Content-Type','application/json; charset=UTF-8');
        return this.http.post(url,json,headers )
            .map(res => res.json());
    }
}

这是运行该方法的代码

logIn(user: string,password: string){
      this.apiService.login(user,password).subscribe(
          data => console.log(data),error => console.log(error)
      );
    }