I ran their code and discovered that it passes if the test is modified from assert.strictEqual to assert.equal, if that helps.

Please when reporting an issue, provide the code that gets that error. About what you said, change Node version to another one, and you'll see you're returning a function instead of a string in that test. I agree the error message in Node v18 is confusing, but I don't know why it shows that, so I'll leave the issue open just in case anyone else knows how to fix that.