Complex object Mapping returns null if First column value null

From the query

SELECT
B.BookId, b.BookName, b.Rate,
a.AuthorId,
a.AuthorName,
a.StreetName,
a.DoorNo,
a.AddressLine1,
a.AddressLine2,
a.District
FROM Book b INNER JOIN Author a
ON b.AuthorId = a.AuthorId
below is the dataset.

With dapper, populate Book, Author and Address complex objects.

Book:

public class Book
    {
        public long BookId { get; set; }
        public string Name { get; set; }
        public Author Author { get; set; }
        public decimal Rate { get; set; }
    }

Author

public class Author
    {
        public long AuthorId { get; set; }
        public string AuthorName { get; set; }
        public Address Address { get; set; }
    }

Address

public class Address
    {
        public string StreetName { get; set; }
        public string DoorNo { get; set; }
        public string AddressLine1 { get; set; }
        public string AddressLine2 { get; set; }
        public string District { get; set; }
    }

Below is the dapper implementation.

await _connection.QueryAsync(@"SELECT 
B.BookId, 
b.BookName, 
b.Rate, 
a.AuthorId,
a.AuthorName,
a.StreetName,
a.DoorNo,
a.AddressLine1,
a.AddressLine2,
a.District
FROM Book b INNER JOIN Author a 
ON b.AuthorId = a.AuthorId",
    new[] { typeof(Book), typeof(Author), typeof(Address) },
    (responseObj) =>
    {
        var book = responseObj[0] as Book;
        if (book != null)
        {
            var author = responseObj[1] as Author;
            if (author != null)
            {
                author.Address = responseObj[2] as Address;
            }
            book.Author = author;
        }
        return book;
    },
    splitOn: "BookId, AuthorId, StreetName");

After mapping below is the response.

Start by reproducing the provided SQL query with Dapper's multi-type QueryAsync mapping and splitOn values, including a null first-column value. Compare the Book, Author, and Address results with the reported response. Done means the intended nested objects are mapped correctly in this case and the behavior is covered by a regression test.