E11000重复键错误索引：id

Question

我已经创建了一个工作正常的api，我通过post requet添加了一个用户但是当我尝试添加另一个具有不同id的用户时我得到了这个错误：

SUCCESS: {
    code = 11000;
    errmsg = "E11000 duplicate key error index: matchmaking.users.$id_1 dup key: { : null }";
    index = 0;
    op =     {
        "__v" = 0;
        "_id" = 352523;
        birthday = "1998-10-09T23:00:00.000Z";
        country = denmark;
        name = Frederik;
    };
}

UserSchema

var userSchema = new Schema({
  _id: {
    type: SchemaTypes.Long,
    required: true,
    unique: true
  },
  accessToken: String,
  email: String,
  name: String,
  birthday: Date,
  country: String,
  created_at: Date,
  updated_at: Date
});