remix – Understanding Revert with Array of Contracts

I have the following solidity file:

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

error MustBeOwner();
error BookDoesNotExist();

// book contract
contract Book {
    bool public isRegistered = false;
    string public title;
    string public author;
}

// actual library
contract Library {

    address owner;

    // all books
    Book[10] books;


    // rented books
    mapping(address => uint) rentedBooks;


    constructor() {
        owner = msg.sender;
    }


    function rentBook(uint _bookId) public view {
        if(msg.sender != owner) {
            revert MustBeOwner();
        }

        if(_bookId > books.length || books[_bookId].isRegistered()) {
            revert BookDoesNotExist();
        }
    }

}

I insert it in Ethereum Remix. It compiles successfully. I deploy it, and run function rentBook(6). A very vague error message is shown:

transact to Library.rentBook errored: VM error: revert.

revert
The transaction has been reverted to the initial state.

Note: The called function should be payable if you send value and the value you send should be less than your current balance.
Debug the transaction to get more information.