C# ado stored procedure (return messgae)

I am trying to make a simple asp.net page that shows a grid view, textbox to enter a new item and a button that submits it through a stored procedure and populates an output parameter in a label. I am right now getting an error:

Conversion failed when converting the varchar value 'test4 has been submitted successfully' to data type int

I am very new to programming so all suggestions are welcome. I havent found a tutorial for using varchar so if you know of one let me know

=============
Storedprocedure
=============
ALTER PROCEDURE [dbo].[Add_Addresstype]
-- Add the parameters for the stored procedure here
@t_Addresstype VarChar(50),
@t_return VarChar(255) OUTPUT

AS
BEGIN
IF EXISTS (SELECT t_AddressType From CM_AddressType WHERE t_AddressType = @t_Addresstype)
BEGIN
SET @t_return = 'The Address Type ' + @t_AddressType + ' already exists!'
RETURN @t_return
END
ELSE
BEGIN
INSERT INTO CM_AddressType (t_addresstype) VALUES (@t_AddressType)
Set @t_return = @t_AddressType + ' has been submitted successfully'
RETURN @t_return
END
END
=============
c# Code
=============
SqlCommand cmd = new SqlCommand("Add_AddressType", CRMConnectionString);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@t_AddressType", txtAddressType.Text);
cmd.Parameters["@t_AddressType"].Value = txtAddressType.Text;
cmd.Parameters.Add(new SqlParameter("@t_return", SqlDbType.VarChar, 255));
cmd.Parameters["@t_return"].Direction = ParameterDirection.Output;
CRMConnectionString.Open();
cmd.ExecuteNonQuery();
CRMConnectionString.Close();
string t_return = cmd.Parameters["@t_return"].Value.ToString();
lblreturn.Text = t_return;

Strange.....I commented ou the RETURN lines and it works now. i get my message and everything!

How would i refresh my gridview after the procedure is finisheD?