Postgres adapter misconfigured on Linux?

I’ve written a simple Rails migration that adds an index to an existing Postgres table:

class AddIndexToEvents < ActiveRecord::Migration
def change
add_index :sf_events, :account_id

However, when I run the migration, it fails due to a syntax error on the CREATE INDEX line. I’m expecting some SQL like this to be generated:

CREATE INDEX index_sf_events_on_account_id ON sf_events (account_id);

But instead I get something like this:

CREATE INDEX index_sf_events_on_account_id ON sf_events ( );

The migration works fine on my friend’s Mac, but it fails on my Linux box. It seems like I may have the wrong/outdated packages. I’m running Ubuntu 12.10 and have these packages installed: libpq-dev, libpq5, postgresql, postgresql-9.1, postgresql-client-9.1, postgresql-client-common, postgresql-common, postgresql-contrib, postgresql-contrib-9.1.

Here’s the full error message:

No Postgres adapter was broken. It turns out that another developer had installed the postgres_ext gem, which doesn’t support add_index. Removing that gem fixed the failures.