不能将空值分配给类型

在我的typescript项目中,我创建了一个自定义类(灵感来自rust's Result<T, E> (顺便说一句)看起来是这样的:

export class Result<T, E extends Error = Error>
{
    public errors: E[] = [];
    public result: T | null = null;

    public static ok<T, E extends Error>(result: T): Result<T, E>
    {
        return new Result(result, [] as E[]);
    }

    public static err<T, E extends Error>(...errors: E[]): Result<T, E>
    {
        return new Result<T, E>(null as unknown as T, errors);
    }

    public static apiErr<T>(...errors: ApiErrorKind[]): Result<T, ApiError>
    {
        return new Result<T, ApiError>(null as unknown as T, errors.map(e => new ApiError(e)));
    }

    private constructor(result: T | null, errors: E[])
    {
        this.result = result;
        this.errors = errors;
    }

    public and<T2, E2 extends Error = E>(f: (x: T) => Result<T2, E2>): Result<T2, E | E2>
    {
        if (this.isErr)
            return Result.err(...this.errors);

        let errors: (E | E2)[] = this.errors;

        try
        {
            return f(this.result as T);
        }
        catch (e)
        {
            errors.push(e as E2);
        }

        return Result.err(...errors);
    }

    // irrelevant methods omitted for brevity

    public then<T2, E2 extends Error = E>(f: (x: T | null) => T2): Result<T2, E | E2>
    {
        if (this.isErr)
            return Result.err(...this.errors);

        let errors: (E | E2)[] = this.errors;

        try
        {
            return Result.ok(f(this.result));
        }
        catch (e)
        {
            errors.push(e as E2);
        }

        return Result.err(...errors);
    }
}

然而,每当我尝试使用它时,我总是会得到关于空值转换的神秘错误,尽管没有任何错误 null 在望:

export async function createUser(input: UserInput, organization: string): Promise<Result<User, ApiError>>
{
    const validation = validateUser(input); // returns Result<boolean, ApiError>

    if (validation.isErr)
        return Result.err<User, ApiError>(... validation.errors); // Error on this line
}

TS23 22 类型“user null”不能分配给类型“result”。类型“null”不能分配给类型“result”。

为什么会这样?