打字的猫鼬?

我有一个nodejs和typescript的项目。我正在使用mongoose连接mongoDb数据库。我的代码是这样的

import { Schema, Document, Model } from 'mongoose';
import * as mongoose from 'mongoose';

export interface IProblem extends Document {
  problem: string;
  solution: string;
}

const ProblemSchema = new Schema({
  problem: { type: String, required: true },
  solution: { type: String, required: true },
});

export async function findOneByProblem(
  this: IProblemModel,
  { problem, solution }: { problem: string; solution: string }
): Promise<IProblem> {
  const record = await this.findOne({ problem, solution });
  return record;
}

export default mongoose.model('Problem', ProblemSchema);

ProblemSchema.statics.findOneByProblem = findOneByProblem;

export interface IProblemModel extends Model<IProblem> {
  findOneByProblem: (
    this: IProblemModel,
    { problem, solution }: { problem: string; solution: string }
  ) => Promise<IProblem>;
}

然而,在这些线路上

const record = await this.findOne({ problem, solution });
return record;

我听到一个编译器错误这样说

TS2322: Type 'IProblem | null' is not assignable to type 'IProblem'.   Type 'null' is not assignable to type 'IProblem'.