C# - Adding Own Control To The Toolbox

Does anyone know why I can't add a subclass that inherits a control from AjaxControlToolKit to the ToolKit? I explicitly implemented IComponent which I didn't really have to since the IControl in the parent implements IComponent already. I'll compile the code, and try to add the DLL but I get the "doesn't contain any controls" error. I know this is a very specific error so there wasn't much help elsewhere.

[System.ComponentModel.DesignerCategory("Component"),
ToolboxData("<{0}:TabPanelWithDataBinding runat=server></{0}:TabPanelWithDataBinding>")]
public class TabPanelWithDataBinding : TabPanel, IComponent, IDisposable
{
//all the meat stuff
}