When I try to use type narrowing on a private member, I get a typescript error

You can see an example below.

class Inventory {
    private book?: string;

    hasBook(): this is { book: string } {
        return !!this.book;
    }

    // Example method
    bookLength(): number | undefined {
        const hasBook = this.hasBook();
        if (hasBook) {
            // TypeScript knows that `this.scmAdapter` exists here
            return this.book.length;
        } else {
            return undefined;
        }
    }
}

The line return this.book.length; will show a typescript error

Property 'book' does not exist on type 'never'.
  The intersection 'this & { book: string; }' was reduced to 'never' because property 'book' exists in multiple constituents and is private in some.

I really want to use type narrowing and avoid using null assertion or optional chaining or other workarounds.

Is there a way to get this to work that I am missing?