Incompatible Web Part markup detected. Use ".dwp web part XML instead of *.webpart web part xml

I'm on MOSS 2007 and just developed this web part deriving from Microsoft.SharePoint.WebParPages.WebPart.  It works fine if I manually deploy it to MOSS, that is, manually copy the DLL and add the SafeControl entry into web.cnofig, and then add it in the web part gallery.   

If I package it up in a solution .wsp file,  adding the solution, deploying it, and activating it all work fine.  However, when I go to the home page and try to add it, I get this popup js alert message:

Unable to add selected web part(s).

<web part name>: Incompatible Web Part markup detected.  Use ".dwp web part XML instead of *.webpart web part xml.

 

_________________________________________________

How i solved it?

 

I fixed it by inheriting from

System.Web.UI.WebControls.WebParts.WebPart and deployed it successfully.