Rails Atom builder xml.tag!("activity:object-type") error

Question

I am creating a activity stream of my feeds according to the http://activitystrea.ms standard.

I am using rails atom builder to create a valid atom feed, but when I use

xml.tag!("activity:object-type", "http://activitystrea.ms/schema/1.0/person")

the atom feed goes blank.On seeing the source I find the correct xml format

from what I have found out the ":object" is giving the problem.