C#调用存储过程出错：String[2]: Size 属性具有无效大小值 0

Create PROCEDURE [dbo].[TestPR]
	@RequestId int,
	@Sender nvarchar(20) output,
	@Receiver nvarchar(20) output,
	@MsgTitle nvarchar(100) output,
	@MsgContent nvarchar(500) output,
AS
GO

cmd.Parameters.Add(new SqlParameter("@RequestId", SqlDbType.Int));
cmd.Parameters.Add(new SqlParameter("@Sender", SqlDbType.NVarChar[color=Red], 20[/color]));
cmd.Parameters.Add(new SqlParameter("@Receiver", SqlDbType.NVarChar[color=Red], 20[/color]));
cmd.Parameters.Add(new SqlParameter("@MsgTitle", SqlDbType.NVarChar[color=Red], 100[/color]));
cmd.Parameters.Add(new SqlParameter("@MsgContent", SqlDbType.NVarChar[color=Red], 500[/color]));