Cannot drag a new Custom Control onto Designer

[Mark Sisson]

Simple question but I'm considering hari-kari:

I created a CustomControl starting from a c# Web Control Library
Project. I immediately compiled it in Release mode without touching a
line of code. Next, I went to another instance of VS.NET where I have
a web page up in design mode. I went to the Toolbox, selected
CustomizeToolbox, and browsed to add my new WebControlLibrary1.dll.
The default cog icon showed up for my WebCustomControl1 but then when
I went to drag it onto the designer I couldn't. All I got when I
moved the mouse over the designer was the "cannot drop here" icon.

Any ideas? Why can't I drop a plain vanilla custom control onto the
design??????