oracle ExecuteScalar在并行编程中有时会返回null

我使用并行编程来执行两个并行的oracle数据库查询。有时"res"对象返回null。知道为什么吗?请注意，这是对同一个表的计数查询，没有任何东西修改这个表，所以数据库表和查询都没有改变。

string query = "select count(*) from SALES_ADVENTUREWORKS2012.SALESORDERDETAIL where PRODUCTID=709";
tasks[0] = Task.Factory.StartNew(() => db.ExecScalarQuery(query));
tasks[1] = Task.Factory.StartNew(() => db.ExecScalarQuery(query));
Task.WaitAll(tasks);
//---------------
public void ExecScalarQuery(String query)
{
    OracleConnection conn = new OracleConnection(connectionString);
    try
    {
        conn.Open();
        OracleCommand cmd = new OracleCommand();
        cmd.Connection = conn;
        cmd.CommandText = query;// "select count(*) from SALES_ADVENTUREWORKS2012.SALESORDERDETAIL where PRODUCTID=709";
        cmd.CommandType = CommandType.Text;
        cmd.CommandTimeout = QUERY_TIMEOUT;
        Object res cmd.ExecuteScalar();
    }
    finally
    {
        conn.Close();
    }
}