Howto: Fix "Invalid Driver Specified" of Armitage on Backtrack 5

After I try to use Armitage in Backtrack 5 Gnome 64 Bit. It's say "Invalid Driver Specified". I try to search how to fix this error and I get this instruction.

1. apt-get install rubygems
2. gem install mysql
3. gem update

Now, try to run Armitage again. The error is gone.