charset=us-ascii mandatory?

Is it necessary to specify the parameter "charset=us-ascii"
when a document contains only ASCII characters?
Currently, the validator refuses to validate such documents
and complains "no character encoding found".
Isn't "us-ascii" the default value for "charset"?

-- 
In memoriam Alan J. Flavell
http://groups.google.com/groups/search?q=author:Alan.J.Flavell