Personal Library Failing 3rd test

Tell us what’s happening:
I am passing all test, but the third one. My solution is very close to what is being asked for in the third test so I am having some trouble finding out why it is failing.

I am getting an error in my console when I run the test:

frame-runner.ts:128 Error: expected null to be a string
    at eval (eval at <anonymous> (frame-runner.961c324637fc97bd00f0.js:2), <anonymous>:21:11)

I am not aware if this has any relevance.

The requirements of the test I am failing:

You can send a GET request to /api/books and receive a JSON response representing all the books. The JSON response will be an array of objects with each object (book) containing title , _id , and commentcount properties.

Your project link(s)

solution: https://replit.com/@caryaharper/boilerplate-project-library
solution-website:https://boilerplate-project-library.caryaharper.repl.co
FCC-solution-website: https://personal-library.freecodecamp.rocks/

Your browser information:

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36

Challenge: Personal Library

Link to the challenge: