Getting Error When Retrieving Data From Stored Procedure

Hi,

I'm at my wit's end with this. I have a simple stored procedure:

PROCEDURE [dbo].[PayWeb_getUserProfile] ( @user_id_str varchar(36), @f_name varchar(50) OUTPUT)
AS

BEGIN
DECLARE @user_id uniqueidentifier;

SELECT @user_id = CONVERT(uniqueidentifier, @user_id_str);

select @f_name=f_name FROM dbo.tbl_user_profile WHERE user_id = @user_id;

END
RETURN

The proc runs perfectly from the Management Console. In my ASP.net form (C#), the following always results in: "Invalid attempt to read when no data is present." (I'll highlight the line where the error results):

SqlConnection conn = new SqlConnection(<connection string data>);
SqlDataReader reader;

SqlCommand comm = new SqlCommand("dbo.PayWeb_getUserProfile", conn);
comm.CommandType = System.Data.CommandType.StoredProcedure;

SqlParameter f_name = new SqlParameter("@f_name", SqlDbType.VarChar, 50);

f_name.Direction = ParameterDirection.Output;
comm.Parameters.Add(f_name);

comm.Parameters.Add("@user_id_str", SqlDbType.VarChar, 36).Value = Membership.GetUser().ProviderUserKey.ToString().ToUpper();
reader = comm.ExecuteReader();
Response.Write("Num: " + reader["@f_name"].ToString()); // ERROR: Invalid attempt to read when no data is present.

reader.Close();
conn.Close();

The data is there. When I put a watch on 'Reader', I see the correct first_name data there.

Your help is appreciated.