这段代码不起作用：

using (var conn = new OracleConnection(ConnString))
{
    conn.Open();
    using (var t = conn.BeginTransaction())
    {
        var sanitizer = new HtmlSanitizer();

        var sqlWithSubquery = @"INSERT INTO owner.dummy_table (idDog, idCat, idBunny) VALUES (
            SequenceDogs.nextval, 
            :Parameter_IdCat, 
            (SELECT idBunny FROM owner.bunny_table WHERE
            IdTree = :Parameter_IdTree))";

        var cmdInsertWithSubquery = new OracleCommand(sqlWithSubquery.ToString(), conn);
        cmdInsertWithSubquery.Parameters.Add(new OracleParameter("Parameter_IdCat", OracleDbType.Int32)).Value = sanitizer.Sanitize(idCat);
        cmdInsertWithSubquery.Parameters.Add(new OracleParameter("Parameter_IdTree", OracleDbType.Int32)).Value = sanitizer.Sanitize(idTree);
        cmdInsertWithSubquery.CommandType = System.Data.CommandType.Text;

        var result = cmdInsertWithSubquery.ExecuteNonQueryAsync();
        if (result.Status == TaskStatus.Faulted)
        {
            sbLog.AppendLine(result.Exception.InnerException.Message.ToString());
        }
        else
        {
            sbLog.AppendLine("Success");
        }

        t.Commit();

        conn.Close();
    }
}

它会导致错误：ORA-01400：无法将NULL插入（“owner”.“dummy_table”.“idBunny”）
而下面的工作刚刚好：

using (var conn = new OracleConnection(ConnString))
{
    conn.Open();
    using (var t = conn.BeginTransaction())
    {
        var sanitizer = new HtmlSanitizer();

        // Doing the subquery apart, because for some reason it won't work inside an INSERT
        var sqlSubquery = @"SELECT idBunny FROM owner.bunny_table WHERE
            IdTree = :Parameter_IdTree";
        var cmdSubquery = new OracleCommand(sqlSubquery.ToString(), conn);
        cmdSubquery.Parameters.Add(new OracleParameter("Parameter_IdTree", OracleDbType.Int32)).Value = sanitizer.Sanitize(idTree);
        cmdSubquery.CommandType = System.Data.CommandType.Text;
        var idBunny = cmdSubquery.ExecuteScalar();


        var sqlInsert = @"INSERT INTO owner.dummy_table (idDog, idCat, idBunny) VALUES (
            SequenceDogs.nextval, 
            :Parameter_IdCat, 
            :Parameter_IdBunny)";
        var cmdInsert = new OracleCommand(sqlInsert.ToString(), conn);
        cmdInsert.Parameters.Add(new OracleParameter("Parameter_IdCat", OracleDbType.Int32)).Value = sanitizer.Sanitize(idCat);
        cmdInsert.Parameters.Add(new OracleParameter("Parameter_IdBunny", OracleDbType.Int32)).Value = sanitizer.Sanitize(idBunny.ToString());
        cmdInsert.CommandType = System.Data.CommandType.Text;

        var result = cmdInsert.ExecuteNonQueryAsync();
        if (result.Status == TaskStatus.Faulted)
        {
            sbLog.AppendLine(result.Exception.InnerException.Message.ToString());
        }
        else
        {
            sbLog.AppendLine("Success");
        }

        t.Commit();

        conn.Close();
    }
}

问题解决了，但它看起来像一个丑陋的解决方案，我想知道为什么第一种方法是无效的。
是绑定过程中的一些bug还是设计的一个特性？