Initializing from irb succeeds, initializing from script fails

Initializing from irb succeeds, initializing from script fails

I'm trying to initialize a MongoClient from a script but I am failing.
What I simply do is:
require 'mongo'
include Mongo
client = MongoClient.new("localhost", 27017)
and it fails with the following error:
/var/lib/gems/1.9.1/gems/mongo-1.9.1/lib/mongo/cursor.rb:43:in
`initialize': undefined method `connection' for nil:NilClass
(NoMethodError)
from /var/lib/gems/1.9.1/gems/mongo-1.9.1/lib/mongo/db.rb:564:in `new'
from /var/lib/gems/1.9.1/gems/mongo-1.9.1/lib/mongo/db.rb:564:in
`command'
from
/var/lib/gems/1.9.1/gems/mongo-1.9.1/lib/mongo/mongo_client.rb:591:in
`block in check_is_master'
from /usr/lib/ruby/1.9.1/timeout.rb:68:in `timeout'
from
/var/lib/gems/1.9.1/gems/mongo-1.9.1/lib/mongo/mongo_client.rb:590:in
`check_is_master'
from
/var/lib/gems/1.9.1/gems/mongo-1.9.1/lib/mongo/mongo_client.rb:474:in
`connect'
from
/var/lib/gems/1.9.1/gems/mongo-1.9.1/lib/mongo/mongo_client.rb:698:in
`setup'
from
/var/lib/gems/1.9.1/gems/mongo-1.9.1/lib/mongo/mongo_client.rb:155:in
`initialize'
from ./test.rb:52:in `new'
from ./test.rb:52:in `<main>'
But, when I try the exact same thing from irb, it works.
Ruby version is:
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]
irb is:
/usr/bin/irb1.9.1 --version irb 0.9.6(09/06/30)
I also tried requiring 'rubygems', but from what i know, Ruby 1.9 doesn't
need it. Am I forgetting something?