this.state.fotos.map is not a function

quando faço o login está retornando esse erro.

TypeError: this.state.fotos.map is not a function Timeline.render C:/Users/dozv/Desktop/Alura/Engenheiro Front-End/React/instalura/src/componentes/Timeline.js:21 18 | 19 | render(){ 20 | return (

21 |

| ^ 22 | { 23 | this.state.fotos.map(foto => ) 24 | }Timeline.js ` import React, { Component } from 'react'; import FotoItem from './Foto';

export default class Timeline extends Component {

constructor(){
  super();
  this.state = {fotos:[]};
}

componentDidMount(){
  fetch(`https://instalura-api.herokuapp.com/api/public/fotos?X-AUTH-TOKEN=${localStorage.getItem('auth-token')}`)///rafael
   .then(response => response.json())
   .then(fotos => {
     this.setState({fotos:fotos});
   });
}

render(){
    return (
    <div className="fotos container">
      {
        this.state.fotos.map(foto => <FotoItem key={foto.id} foto={foto}/>)
      }                
    </div>            
    );
}

}

Login.js

import React, { Component } from 'react'; import {browserHistory} from 'react-router';

export default class Login extends Component {

constructor(props){
    super(props);        
    this.state = {msg:this.props.location.query.msg};
}

envia(event){
    event.preventDefault();

    const requestInfo = {
        method:'POST',
        body:JSON.stringify({login:this.login.value,senha:this.senha.value}),
        headers:new Headers({
            'Content-type' : 'application/json' 
        })
    };

    fetch('https://instalura-api.herokuapp.com/api/login',requestInfo)
        .then(response => {
            if(response.ok) {
                return response.text();
            } else {
                throw new Error('não foi possível fazer o login');
            }
        })
        .then(token => {
            localStorage.setItem('auth-token',token);
            browserHistory.push('/timeline');
        })
        .catch(error => {
            this.setState({msg:error.message});
        });
}

render(){
    return (
        <div className="login-box">
            <h1 className="header-logo">Instalura</h1>
            <span>{this.state.msg}</span>
            <form onSubmit={this.envia.bind(this)}>
                <input type="text" ref={(input) => this.login = input}/>
                <input type="password" ref={(input) => this.senha = input}/>
                <input type="submit" value="login"/>
            </form>
        </div>
    );
}

} ` alguém teve algum problema parecido? OBS estou usando as APIs no herokuapp