Hmm - Unique Id Is In Binary Data - How Do I Query Against It?

Hi - I've got a table in SQL server that has its unique ID field as binary data. I have a gridview displaying data from this table and I have set the datakey of this gridview as that binary data field.

I have a 'Select' ciolumn in my gridview that, when selected, will display more details from the selected record. The problem is, how do I pass the selected id back to sql server bearing in mind that it's binary data? Here's what I'm doing at the moment:

I have a function in my data accsess class that queries the database:

Public Function getDetailsById(ByVal Id As System.Byte) As DataSet

        Dim con As New SqlConnection(conStr)
        Dim cmd As New SqlCommand("SELECT * FROM tableName WHERE Id=@Id", con)

        cmd.Parameters.Add("@Id", SqlDbType.Binary)
        cmd.Parameters("@Id").Value = Id

        Dim ds As New DataSet
        Dim adp As New SqlDataAdapter(cmd)
        adp.Fill(ds)

        Return ds

    End Function

And then I call this function from my page with the gridview using:

 Dim da As New myDataAccess
        Dim ds As New DataSet
        ds = da.getDetailsById(GridView1.SelectedDataKey.Value)


But I get the error:

Conversion from type 'Byte()' to type 'Byte' is not valid.

Any ideas where my data/code is going wrong? I'm not sure how to pass this sort of data around?

Thanks