CLR TVF Error: Msg 6260, Level 16, State 1, Line 1

Dear All, I always got this error in CLR TVF:

Msg 6260, Level 16, State 1, Line 1
An error occurred while getting new row from user defined Table Valued Function :
System.InvalidOperationException: Invalid attempt to FieldCount when reader is closed.
System.InvalidOperationException:
at System.Data.SqlClient.SqlDataReaderSmi.get_FieldCount()
at System.Data.Common.DbEnumerator.BuildSchemaInfo()
at System.Data.Common.DbEnumerator.MoveNext()

Here is my code:

using System;
using System.Data;
using System.Data.Common;
using System.Data.SqlTypes;
using System.Data.SqlClient;
using Microsoft.SqlServer.Server;
using System.Collections;

public static class UserDefinedFunctions
{
[Microsoft.SqlServer.Server.SqlFunction(FillRowMethodName = "rowfiller",DataAccess=DataAccessKind.Read,TableDefinition = "ActID int, ActName nvarchar(50), ActCreatorID int,ActDesp nvarchar(200),ActCreateDate datetime,ActModifyDate datetime, ActStartDate datetime, ActEndDate datetime, Status int, Cost int")]
public static IEnumerable Func_GetSchCatActivityIDTable(int CatActivityID)
{
using (SqlConnection connection = new SqlConnection("context connection=true"))
{
string sqlstring = "select * from Activity where CatActivityID=@CatActivityID;";

connection.Open();
SqlCommand command = new SqlCommand(sqlstring, connection);
command.Parameters.AddWithValue("@CatActivityID", CatActivityID);


return command.ExecuteReader(CommandBehavior.CloseConnection);

}
}
[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft Performance","CA1811:AvoidUncalledPrivateCode")]
public static void rowfiller(Object obj,
out SqlInt32 ActID,
out SqlString ActName,
out SqlInt32 ActCreatorID,
out SqlString ActDesp,
out SqlDateTime ActCreateDate,
out SqlDateTime ActModifyDate,
out SqlDateTime ActStartDate,
out SqlDateTime ActEndDate,
out SqlInt32 Status,
out SqlInt32 Cost,
)
{

SqlDataRecord row = (SqlDataRecord)obj;
int column = 0;

ActID = (row.IsDBNull(column)) ? SqlInt32.Null : new SqlInt32(row.GetInt32(column)); column++;
ActName = (row.IsDBNull(column)) ? SqlString.Null : new SqlString(row.GetString(column)); column++;
ActCreatorID = (row.IsDBNull(column)) ? SqlInt32.Null : new SqlInt32(row.GetInt32(column)); column++;
ActDesp = (row.IsDBNull(column)) ? SqlString.Null : new SqlString(row.GetString(column)); column++;
ActCreateDate = (row.IsDBNull(column)) ? SqlDateTime.Null : new SqlDateTime(row.GetDateTime(column)); column++;
ActModifyDate = (row.IsDBNull(column)) ? SqlDateTime.Null : new SqlDateTime(row.GetDateTime(column)); column++;
ActStartDate = (row.IsDBNull(column)) ? SqlDateTime.Null : new SqlDateTime(row.GetDateTime(column)); column++;
ActEndDate = (row.IsDBNull(column)) ? SqlDateTime.Null : new SqlDateTime(row.GetDateTime(column)); column++;
Status = (row.IsDBNull(column)) ? SqlInt32.Null : new SqlInt32(row.GetInt32(column)); column++;
Cost = (row.IsDBNull(column)) ? SqlInt32.Null : new SqlInt32(row.GetInt32(column)); column++;
}


};

Can anyone tell me what I am doing wrong? Many thanks

.