未处理的拒绝（错误）React JS Redux

Question

未处理的拒绝（错误）：给定操作“SET_CARS”，reducer“cars”返回undefined。要忽略某个操作，您必须显式返回先前的状态。如果您希望此reducer不保留任何值，则可以返回null而不是undefined。我如何解决这个问题我不知道我哪里出错了？

动作：index.js

export const SET_CARS = 'SET_CARS';

export function setCars(items){
    return {type: SET_CARS, 
         items}
}

import React, { Component} from "react";
import { Form, FormGroup,FormControl,Button } from "react-bootstrap";
import { connect } from "react-redux";
import { setCars } from "../actions";

class SearchCars extends Component {
  constructor() {
    super();
    this.state = {
      tansmition: ""
    }
  }

  search() {
    let { tansmition } = this.state;
    const url = `http://localhost:4000/vehicles/?transmission=${tansmition}`;
    
    fetch(url, {
      method: 'GET'
    })
    .then(response => response.json())
    .then(json => {
        return this.props.setCars(json.results)
      })
    }


  render() {
    return ( <Form inline >
      < FormGroup >
      < FormControl type = "text" placeholder = "Automated"onChange = { event => this.setState({ tansmition: event.target.value })
      }/> 
      </FormGroup>
       <Button onClick = {
        () => this.search()}> Submit </Button>
       </Form>
    );
  }
}

export default connect(null, {setCars})(SearchCars);

Reducers：index.js

import {SET_CARS} from "../actions";
import {combineReducers} from "redux";

function cars( state = [], action) {
  switch (action.type) {
    case SET_CARS:
      return action.items;
    default:
      return state;
  }
}

const rootReducer = combineReducers({cars});

export default rootReducer;