[Next.js] "unhandledRejection: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client" 오류

https://cocoon1787.tistory.com/714

[Node.js] "UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

//put : http://localhost:3000/api/users/1
export async function PutUsers(req, res) {
  try {
    const { userId } = req.query;
    const formData = req.body;

    if (userId && formData) {
      const user = await Users.findByIdAndUpdate(userId, formData);
      res.status(200).json(user);
    } 
    res.status(404).json({ error: "User Not Selected...!" });
    }
  } catch (error) {
    res.status(404).json({ error: "error while updating the data" });
  }
}

해당과 동일한 코딩시 해당 에러 발생

//put : http://localhost:3000/api/users/1
export async function PutUsers(req, res) {
  try {
    const { userId } = req.query;
    const formData = req.body;

    if (userId && formData) {
      const user = await Users.findByIdAndUpdate(userId, formData);
      res.status(200).json(user);
    } else {
      return res.status(404).json({ error: "User Not Selected...!" });
    }
  } catch (error) {
    res.status(404).json({ error: "error while updating the data" });
  }
}

else 와 return을 추가하였더니 에러가 사라 졌다