TypeError: Cannot read properties of undefined (reading '1')

When trying to mock the post call with Jest, I am getting error as:

TypeError: Cannot read properties of undefined (reading '1')

What is the cause of this? Here is the spect code:

 const mockFn = jest.fn();
  axios.post = mockFn;
  userEvent.click(button);

  const apiCall = mockFn.mock.calls[0];
  const body = apiCall[1];
  expect(body).toEqual({ userName: "arif", password: "one", email: "xyz@abc.com" });

template:

const submit = (event: React.MouseEvent<HTMLButtonElement, MouseEvent>) => {
    event.preventDefault();
    axios.post("/api/1.0/users", formData);
    setFormData(initialFormProps);
  };

html:

<form action="" autoComplete="off">
        <h1>Sign Up</h1>
        <input type="text" placeholder="userName" onChange={handleChange} defaultValue={formData.userName} name="userName" />
        <input type="email" placeholder="userEmail" onChange={handleChange} defaultValue={formData.userEmail} name="userEmail" />
        <input type="password" defaultValue={formData.userPassword} onChange={handleChange} placeholder="userPassword" name="userPassword" />
        <input type="password" defaultValue={formData.userPasswordRep} onChange={handleChange} placeholder="userPassword Repeat" name="userPasswordRep" />
        <button disabled={isSubmit} onClick={submit}>
          Sign Up{" "}
        </button>
      </form>

testing part:

it("sends username, email and password to backend after clicking submit", async () => {
      render(<SignUpPage />);
      const password = screen.getByPlaceholderText("userPassword");
      const userName = screen.getByPlaceholderText("userName");
      const email = screen.getByPlaceholderText("userEmail");
      userEvent.type(password, "one");
      userEvent.type(userName, "arif");
      userEvent.type(email, "xyz@abc.com");
      const button = screen.queryByRole("button", { name: "Sign Up" }) as HTMLElement;

      const mockFn = jest.fn();
      axios.post = mockFn;
      userEvent.click(button);

      const apiCall = mockFn.mock.calls[0];
      const body = apiCall[1];
      expect(body).toEqual({ userName: "arif", password: "one", email: "xyz@abc.com" });
    });

it is working fine after i remove jest.mock('axios'); here is the passing test:

it("sends username, email and password to backend after clicking submit", async () => {
  render(<SignUpPage />);
  const password = screen.getByPlaceholderText("userPassword");
  const userName = screen.getByPlaceholderText("userName");
  const email = screen.getByPlaceholderText("userEmail");
  const rePass = screen.getByPlaceholderText("userPassword Repeat");
  userEvent.type(password, "one");
  userEvent.type(userName, "arif");
  userEvent.type(email, "xyz@abc.com");
  userEvent.type(rePass, "one");
  const button = screen.queryByRole("button", { name: "Sign Up" }) as HTMLElement;
  const payload = { userPassword: "one", userName: "arif", userEmail: "xyz@abc.com", userPasswordRep: "one" };
  const mockFn = jest.fn();
  axios.post = mockFn;
  userEvent.click(button);

  const apiCall = mockFn.mock.calls[0];
  const body = apiCall[1];
  expect(body).toEqual(payload);
});