为什么我收到此操作类型不能为未定义的错误？

Question

我正在尝试测试页面上的登录和注销链接，但是每当尝试执行window.dispatch(signup(user))时，都会收到以下错误消息：

redux-logger.js:1 Uncaught Error: Actions may not have an undefined "type" property. Have you misspelled a constant?

我已经仔细检查了动作，对我来说很好：

export const RECEIVE_CURRENT_USER = 'RECEIVE_CURRENT_USER';
export const LOGOUT_CURRENT_USER = 'LOGOUT_CURRENT_USER';
export const RECEIVE_SESSION_ERRORS = 'RECEIVE_SESSION_ERRORS';

export const receiveCurrentUser = user => ({
  type: RECEIVE_CURRENT_USER,
  user
});

export const logoutCurrentUser = () => ({
  type: LOGOUT_CURRENT_USER,
});

export const receiveSessionErrors = errors => ({
  type: RECEIVE_SESSION_ERRORS,
  errors
});

export const login = user => dispatch => (
  APIUtil.login(user).then(user => dispatch(receiveCurrentUser(user)))
);

export const logout = () => dispatch => (
  APIUtil.logout().then(() => dispatch(logoutCurrentUser())),
  err => dispatch(receiveSessionErrors(err.responseJSON))
);

export const signup = user => dispatch => (
  APIUtil.signup(user).then(user => dispatch(receiveCurrentUser(user))), 
  err => dispatch(receiveSessionErrors(err.responseJSON))
);

我还检查了我的reducer的注册情况，并且我认为自己正确导入了操作：

import {RECEIVE_CURRENT_USER, LOGOUT_CURRENT_USER} from '../actions/session_action';

const _nullUser = Object.freeze({
  id: null
});

const sessionReducer = (state = _nullUser, action) => {
  Object.freeze(state);
  switch (action.type) {
    case RECEIVE_CURRENT_USER:
      return {id: action.currentUser.id};
    case LOGOUT_CURRENT_USER:
      return _nullUser;
    default:
      return state;
  }
}

export default sessionReducer;

关于此错误为何突然出现或可能来自何处的任何想法？

更新：尽管出现了错误消息，但似乎用户注册已经通过，并且该用户确实在我的数据库中。但是当我在控制台上执行getState()时，我创建的用户不会出现在状态中。

以下是错误消息的详细图片：