Why do I get the error "Error Rendering Control"

[Sanjay Pais]

I have created a custom control that inherits from the textbox control and
adds a property to it.

When I drop it on a page it renders just fine.

However,

When I drop it into a web user control (ascx) and drop this user control on
my page I get the error
"Error Rendering Control" - AEPLabel1
An unhandled exception has occurred. Unknown server tag 'AEPC:AEPLabel'.

I have created custom tags for the controls called AEPC.

Thanks in advance

Sanjay