Procedure Or Function Has Too Many Arguments Specified

Hi,
I have a simple stored procedure that basically SELECT some data from database. I am calling this stored procedure using a sqldatasource. The first time I try it, it works perfectly fine. But I get the error message "Procedure or function has too many arguments specified" the second time I try to search for thing. This is the code that I have that called the stored procedure:
 Session("SearchItem") = txtSearch.Text.Trim
 SqlDataSource1.SelectCommand = "_MyStoredProcedure"
 SqlDataSource1.SelectParameters.Add("SearchItem", Session("SearchItem"))
 SqlDataSource1.SelectCommandType = SqlDataSourceCommandType.StoredProcedure
 Gridview1.DataSourceID = "SqlDataSource1"
 Gridview1.DataBind()
 The following is the code in the stored procedure:
 CREATE PROCEDURE [dbo].[_MyStoredProcedure]
(
    @SearchItem nvarchar(255) = NULL
)
AS


SELECT Field1, Field2, Field3
FROM Registration

WHERE
((@SearchItem IS NULL)
OR (Field1 LIKE '%' + @SearchItem + '%')
OR (Field2 LIKE '%' + @SearchItem + '%'))

GO
 Please help.
Stan