How to mock a class private variable in typescript

I have a class in which I instantiate an instance of a redis client as a private variable which is set in the constructor but not passed to the constructor. I have two methods I want to unit test and I want to stub the client to throw an error. This is a simple representation of my class.

class MyClass {
    #client;
    #host;
    #port;
    #username;
    #password;

    constructor(options?: MyConfig) {
        options = options || {};
        this.#host = options.host || `localhost`;
        this.#port = options.port || 6389;
        this.#username = options.username || `username`;
        this.#password = options.password || `password`;
        this.#client = createClient({...})
    }

     async #init() {
        try {
            await this.#client.connect();
            const exists = await this.#client.exists(KEY);
            // i need to mock the client to throw an error here
            if(!exists) await this.#client.set(KEY, MY_VALUE);
        } catch (err) {
            throw err;
        }
    }

    async getResult(count?: number): Promise<number> {
        ...
        // i need to mock the client to throw an error here
        this.#client.getScriptResult();
    }
}

I'm using mocha and chai, and added sinon to try mocking with no success, what is the right way to unit test this case?