with mongoid 2.0.0rc7 (but even with previouses rc: i tried to downgrade) i receive this error:

(rails/gem dir)/mongo-1.2.0/lib/mongo/util/support.rb:50: in 'validate_db_name': db_name must be a String or Symbol (TypeError).
just a query is executed:

admin['$cmd'].find({:ismaster=>1}, {}).limit(-1)

My yml file is fine, i even rebuilt it with rails g mongoid:config, i then started a new empty app to try if something was wrong with my system: same error.
I then tried mongo_mapper, and everything works with it. Seems something is wrong with mongoid... does anyone have this problem? Any tip on how to solve it?