ERROR : Enclosing class/module 'mMysql2' for class Client not known

hi everybody ,
i am trying to test a simple webapplication but got the following
error :

molay@molay-VirtualBox:~$ gem install mysql2
Building native extensions. This could take a while...
Successfully installed mysql2-0.2.6
1 gem installed
Installing ri documentation for mysql2-0.2.6...
Enclosing class/module 'mMysql2' for class Client not known
Installing RDoc documentation for mysql2-0.2.6...
Enclosing class/module 'mMysql2' for class Client not known

my data base config is showing the corect info - mysql 2 adapter

any help ?

thanks