Forms Data Controls :: SQL Timestamp In GridView And Manually Added Update Parameter For Stored Procedure In C#

I have a SQL table containing a timestamp column. This table is displayed in ASP using GridView control. The timestamp column is indicated in the DataKeyNames property of the GridView. I'm using TemplateFields for all columns and a ItemTemplate HiddenField control connected to the timestamp column using Eval.

<asp:TemplateField
ItemStyle-Wrap="false"
Visible="false"
HeaderText="timestamp"
SortExpression="timestamp">
<ItemTemplate>
<asp:HiddenField
ID="gv_hid_timestamp"
runat="server"
Value='<%#
Eval("timestamp") %>'
></asp:HiddenField>
</ItemTemplate>
</asp:TemplateField>

The data is displayed properly. I'm trying implement the optimistic concurrency in C# using the value of above mentioned timestamp and a stored procedure. The SQL stored procedure expects the @timestamp as varbinary(8). I'm not sure how to pass the value of gv_hid_timestamp back to the stored procedure. I'm using the following:

command.Parameters.Add("@timestamp", SqlDbType.Binary).Value = Byte.Parse(((HiddenField)gv.Rows[row].FindControl("gv_hid_timestamp")).Value);

resulting in the following error: "Input string was not in correct format." (I also tried different data types but with similar result). I want to be able to pass this parameter programmatically from C# and not have to specify it on the UpdateParameters list.