Mongodb Mongoose：需要验证错误路径

我试图用mongoose在mongodb中保存一个新文档，但我得到了ValidationError: Path 'email' is required., Path 'passwordHash' is required., Path 'username' is required.个，尽管我提供了邮箱、密码哈希和用户名.

这是用户模式.

    var userSchema = new schema({
      _id: Number,
      username: { type: String, required: true, unique: true },
      passwordHash: { type: String, required: true },
      email: { type: String, required: true },
      admin: Boolean,
      createdAt: Date,
      updatedAt: Date,
      accountType: String
    });

这就是我创建和保存用户对象的方式.

    var newUser = new user({

      /* We will set the username, email and password field to null because they will be set later. */
      username: null,
      passwordHash: null,
      email: null,
      admin: false

    }, { _id: false });

    /* Save the new user. */
    newUser.save(function(err) {
    if(err) {
      console.log("Can't create new user: %s", err);

    } else {
     /* We succesfully saved the new user, so let's send back the user id. */

    }
  });

那么为什么mongoose会返回一个验证错误，我不能使用null作为临时值吗？