Error: Sequence contains no matching element in C# Asp.Net

 public async Task<int> MultipleInserBooks(List<BooksInfoEntities> books)
{
    try
    {
        if (books == null || !books.Any()) return 0;
        List<BooksInfoEntities> newBooks = new List<BooksInfoEntities>();
        
        // check duplicate books
        foreach (var book in books)
        {
            if (!string.IsNullOrEmpty(book.BookName))
            {
                var bookObj = newBooks.First(x => x.BookName.Trim().Equals(book.BookName));
                if (bookObj == null)
                {
                    newBooks.Add(book);
                }
            }
        }
        
        // Insert books and return
        return updateBooks(newBooks);
    }
}

 System.InvalidOperationException: Sequence contains no matching element
   at BooksIndexer.Dal.BooksService.BooksServiceDal.<MultipleInserTokenSpam>d__3.MoveNext() in C:\Project\Books\Books.IO\BooksIndexer\BooksIndexer\BooksIndexer\BooksIndexer.Dal\BooksService\BooksServiceDal.cs:line 196
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at BooksIndexer.Bsl.BooksService.BooksServiceBsl.<Run>d__2.MoveNext() in C:\Project\Books\Books.IO\BooksIndexer\BooksIndexer\BooksIndexer\BooksIndexer.Bsl\BooksService\BooksServiceBsl.cs:line 44Round 2 is starting....