add_index fails for composite index

Anyone encountering an error with add_index and multiple columns?

For example -

add_index(:article_views, [:session_id,:ip_address], :unique =>
true, :name => 'article_views_unique')

in a migration throws the following error -

"wrong number of arguments (1 for 0)"

but the following works fine !!

add_index(:article_views, [:session_id], :unique => true, :name =>
'article_views_unique')

The documentation says that the column name that follows the table
name can be an array of symbols, but it seems to accept only one
entry.

Using Rails 2.3.2 against PostgreSQL using the Pg 0.8.0 driver.

Thanks for any help.
Gokul.

Found the problem !!
I had to uninstall the Less gem (less-1.1.12) .

rake db:migration --trace gave me the following stack -

wrong number of arguments (1 for 0)
/usr/lib/ruby/gems/1.8/gems/less-1.1.12/lib/ext.rb:60:in `inspect'
/usr/lib/ruby/gems/1.8/gems/less-1.1.12/lib/ext.rb:60:in `send'
/usr/lib/ruby/gems/1.8/gems/less-1.1.12/lib/ext.rb:60:in `to_proc'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.2/lib/active_record/
migration.rb:346:in `map'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.2/lib/active_record/
migration.rb:346:in `method_missing'
....rest deleted.....