这是我的userRepository特征

import scala.collection.mutable.Map

trait userRepository {
  def addUser(id: String, user:USER):Map[String,USER]
  def removeUser(id: String):Map[String,USER]
  def getUser(id: String):Map[String,USER]
}

下面是我的userRepositoryImpl类

import scala.collection.mutable.Map

class userRepositoryImpl extends userRepository {
  val userMap = Map.empty[String,USER]

  override def addUser(id: String, user: USER): Map[String,USER] = {
    userMap.put(id, user)
  }

  override def removeUser(id: String): Map[String,USER] = {
      userMap.remove(id)
    }

  override def getUser(id: String): Map[String,USER] = {
    userMap.find(_._2.id == id)
  }
}

下面是我的USER类

case class USER(id: String, firstName: String, lastName: String, email: String, address: String, gender: String)

埃罗

type mismatch;
 found   : Option[USER]
 required: scala.collection.mutable.Map[String,USER]
    userMap.put(id, user)

为什么我会得到这个错误？可以任何朋友解释的原因？