Use A Custom Parameter With SqlDataSource?

I want to add a GridView control to one of my views that is backed by a SqlDataSource. The SelectCommand query is parametrized on the aspnet_Users.UserId GUID for the currently-logged-in user, so I need a way to pass the user ID as a parameter.

I read How to utilize ASP.NET current user name in SqlParameter without code-behind and decided to create a custom Parameter named UserIdParameter:

namespace MyApp.Web
{
public class UserIdParameter : Parameter
{
public UserIdParameter(string name)
: base(name)
{
}
protected UserIdParameter(UserIdParameter parameter)
: base(parameter)
{
}

protected override Parameter Clone()
{
return new UserIdParameter(this);
}
protected override object Evaluate(HttpContext context, Control control)
{
return Membership.GetUser().ProviderUserKey;
}
}
}

In the ASPX view I then added:

<%@ Register TagPrefix="my" Namespace="MyApp.Web" %>

in a line after the <%@ Page ... %> line as well as:

<SelectParameters>
<my:UserIdParameter Name="UserId" />
</SelectParameters>

within the asp:SqlDataSource element.

Unfortunately I get a Parser Error ("An error occurred during the parsing of a resource required to service this request. review the following specific parse error details and modify your source file appropriately.") with the following highlighted in red:

<my:UserIdParameter Name="UserId" />

Visual Web Developer 2010 Express is also informing me that "Element 'UserIdParameter' is not a known element. This can occur if there is a compilation error in the Web site, or the web.config file is missing."

Do I need to modify Web.config in some way? If not, what do I need to do to be able to use my custom UserIdParameter parameter?