SPROC Problem - String[1]: The Size Property Has An Invalid Size Of 0.

Hi folks,
Can anyone enlighten me here? I'm trying to use a SPROC which, when supplied with an int, looks up the table and returns certain columns from it. I'm using a SqlCommand, here's my codebehind: 
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SqlCommand dataSource = new SqlCommand("retrieveData", new SqlConnection(dbConnString));

        dataSource .CommandType = CommandType.StoredProcedure;
        dataSource .Parameters.AddWithValue("id", poid);
        dataSource .Parameters.AddWithValue("title", title).Direction = ParameterDirection.Output;
        dataSource .Parameters.AddWithValue("creator", creator).Direction = ParameterDirection.Output;
        dataSource .Parameters.AddWithValue("assignee", assignee).Direction = ParameterDirection.Output;
etc, etc...
And the SPROC:
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set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[retrieveData]
    @id int,
    @title varchar(50) OUTPUT,
    @creator varchar(50) OUTPUT,
    @assignee varchar(50) OUTPUT,
    @contact varchar(50) OUTPUT,
    @deliveryCost numeric(18,2) OUTPUT,
    @totalCost numeric(18,2) OUTPUT,
    @status tinyint OUTPUT,
    @project smallint OUTPUT,
    @supplier smallint OUTPUT,
    @creationDateTime datetime OUTPUT,
    @amendedDateTime datetime OUTPUT,
    @locked bit OUTPUT
   
   
AS
    /**SET NOCOUNT ON;
    **/
    SELECT
    [title] AS [@title], [datetime] AS [@creationDateTime], [creator] AS [@creator], [assignee] as [@assignee],
    [supplier] as [@supplier], [contact] AS [@contact], [delivery_cost] AS [@deliveryCost], [total_cost] AS [@totalCost],
    [amended_timestamp] AS [@amendedDateTime], [locked] AS [@locked]
    FROM purchase_orders
    WHERE [id] = @id;
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 The id being passed in is definately not null, and is set to a value of an item I know exists. The resulting error is:



Exception Details: System.InvalidOperationException: String[1]: the Size property has an invalid size of 0.
Line 63: retrievePODetails.Connection.Open();
Line 64: retrievePODetails.ExecuteNonQuery();[InvalidOperationException: String[1]: the Size property has an invalid size of 0.]
System.Data.SqlClient.SqlParameter.Validate(Int32 index) +717091... ... Can anyone see anything I'm missing? Thanks,Ally