Retrieve Output Parameter From Stored Procedure

Hi,

 I'm having problems retrieving the output parameter from my stored procedure, i'm getting the following error

An SqlParameter with ParameterName '@slideshowid' is not contained by this SqlParameterCollection code as follows:

int publicationId = (int) Session["PublicationId"];
string title = txtTitle.Text;
int categoryid = Convert.ToInt32(dlCategory.SelectedItem.Value);
SqlConnection myConnection = new SqlConnection(ConfigurationSettings.AppSettings["ConnectionString"]);
SqlCommand myCommand = new SqlCommand("sp_be_addSlideshow", myConnection);
myCommand.CommandType = CommandType.StoredProcedure;
myCommand.Parameters.Add(new SqlParameter("@publicationid", SqlDbType.Int));
myCommand.Parameters["@publicationid"].Value = publicationId;
myCommand.Parameters.Add(new SqlParameter("@title", SqlDbType.NVarChar));
myCommand.Parameters["@title"].Value = title;
myCommand.Parameters.Add(new SqlParameter("@categoryid", SqlDbType.Int));
myCommand.Parameters["@categoryid"].Value = categoryid;
myConnection.Open();
myCommand.ExecuteNonQuery();
string slideshowId = myCommand.Parameters["@slideshowid"].Value.ToString();
myConnection.Close();

 my stored procedure:

CREATE PROCEDURE sp_be_addSlideshow
(
 @publicationid int,
 @title nvarchar(50),
 @categoryid int,
 @slideshowid int = NULL OUTPUT
)
AS

INSERT INTO
 Slideshow(publicationid,title,slideshowcategoryid,deleted)

VALUES
 (@publicationid,@title,@categoryid,0)

SELECT @slideshowid = SCOPE_IDENTITY()
GO