Invalid exception message indicated by npgsql

My database script is as follows.

CREATE TABLE books
(
    book_name national character varying(50) not null primary key
);

When I try to execute code in C #

private void button1_Click(object sender, EventArgs e)
{

    string sql = "INSERT INTO books(book_name) SELECT NULL;";
    string connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString;
    using (NpgsqlConnection connection = new NpgsqlConnection(connectionString))
    {
        connection.Open();
        using (NpgsqlCommand npgsqlCommand = new NpgsqlCommand(sql, connection))
        {
            npgsqlCommand.ExecuteNonQuery();
        }
    }

}

This gives me this error:

An unhandled exception of type "System.NotSupportedException" occurred in Npgsql.dll

Additional Information: Backend sent an unrecognized response type: _

I know that I pass a NULL value in a NOT NULL column . I want to get the exact error message. What the hell does the above error mean?