Deleting multiple keys in redis-rb

Using redis-rb in a Rails app, the following doesn't work:

irb> keys = $redis.keys("autocomplete*")
=> ["autocomplete_foo", "autocomplete_bar", "autocomplete_bat"]
irb> $redis.del(keys)
=> 0

This works fine:

irb> $redis.del("autocomplete_foo", "autocomplete_bar")
=> 2

Am I missing something obvious? The source is just:

# Delete a key.
def del(*keys)
  synchronize do
    @client.call [:del, *keys]
  end
end

which looks to me like it should work to pass it an array...?