All controls are null within usercontrol

I have a UserControl which uses a UserControl, among other controls.

In the ascx file I have the following code:

<%@ Register TagPrefix="tag" Namespace="some.name.space" Assembly="some.assembly" %>
<tag:control ID="test" runat="server" />

In my Page_Load method, I try to set a property on test like so:

test.Text = "Hello World!";

This actually sets the Text property of a literal control in my user control test.

This throws an exception:

Object reference not set to an instance of an object

When it tries to set the

lblTest.Text = value; 

The object that is null is lblTest.

Am I not adding the user control correctly? Should I - or do I have to - specify the Src property when registering a Tag? If so, I'd have to register every usercontrol I use?

This also results in no controls loading in usercontrol and all controls are null within usercontrol.