Problem With Two Strings To Be Concatenated Have Different Collation???

Hi all,
I am having a little problem to use CLR Function inside SqlServer 2005.
I do not see any problem with my code in C# as well as SqlServer sides.
Please help if you know the solution.

/************************C# Codes***************************/
 using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

public class ClrUDF {
 [Microsoft.SqlServer.Server.SqlFunction]
 public static SqlString GetString(SqlString str)
 { 
  SqlString tmp = new SqlString("Hello ");
  tmp += str; return tmp;
 }
};

/********************************************************/
CREATE ASSEMBLY [SqlClr] AUTHORIZATION [dbo]
FROM 'C:ProjectsSqlClrSln..... SqlCrl.dll'
WITH PERMISSION_SET = SAFE

Create FUNCTION dbo.GetString(@str as nvarchar(20))
RETURNS nvarchar(200)
AS EXTERNAL NAME SqlClr.ClrUDF.GetString

Try to use the function:

Select dbo.GetString('Jame')

and the error is throw

Msg 6522, Level 16, State 2, Line 1
A .NET Framework error occurred during execution of user defined routine or aggregate 'GetString':
System.Data.SqlTypes.SqlTypeException: Two strings to be concatenated have different collation.
System.Data.SqlTypes.SqlTypeException: at System.Data.SqlTypes.SqlString.op_Addition(SqlString x, SqlString y)
at ClrUDF.GetString(SqlString str)