Element [a user control] is not a known element.

I'm getting errors while trying to compile an ASP.NET app.  This is an app that we converted from .NET 1.1 to 2.0.  The error looks like this:


Element 'ClientSystemMenu' is not a known element. This can occur if there is a compilation error in the Web site.

The element ClientSystemMenu is a user control that we wrote.  It doesn't use anything in ASP.NET AJAX 1.0, and neither does this page that the error is being generated from.

The ClientSystemMenu control is on other .aspx pages and they are not complaining.  So, what is the error about?
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