我在测试 React 应用时遇到此错误： TypeError：expect(...).toBeString 不是函数 。我使用的代码是：

import React from "react";

import { configure, shallow } from "enzyme";
import Adapter from "enzyme-adapter-react-16";
import Footer from "../Footer/Footer";
configure({ adapter: new Adapter() });

describe("<About />", () => {
  it("Check the type of value", () => {
    const props = {
      copyright: "Copyright 2019"
    };
    const wrapper = shallow(<Footer {...props} />);
    expect(wrapper.prop("copyright")).toBeString();
  });
});