"Format is not recognized" error while trying to sign xap-file with signtool

Question

Hi, I am trying to follow this tutotial:
http://channel9.msdn.com/learn/courses/Silverlight4/Overview/Overview/Moving-Beyond-the-Browser--Elevated-Trust-Applications/#_Toc256192962
in order to sign my silverlight application.

But on the last step which is something like that:

signtool sign /v /f c:\Demo\TestOOBCodeSigningCA.pfx /p password c:\Demo\SilverlightApplication2\SilverlightApplication2.Web\ClientBin\SilverlightApplication2.xap

I got the following error:

SignTool Error: This file format cannot be signed because it is not recognized
SignTool Error: An error occured while attempting to sign: c:\Demo\SilverlightApplication2\SilverlightApplication2.Web\ClientBin\SilverlightApplication2.xap

What can be the reason?