Odd "sweep" error on older code

I’ve got a Rails project that has been on the shelf for a few months,
and I’m picking it back up. I’ve frozen Rails at 1.1.4 in vendor/,
and did the “rails .” and “rake rails:update” to get it up to a
current Rails level.

The code initially seems to be working fine, but when I go to my
login method, I get a 500 error thrown. Looking in the
development.log, it seems that it’s a problem with a “sweep” getting
called on a nil. However, I can’t for the life of me figure out what
its problem is.

Here’s the chunk from the log:

Processing AccountController#login (for at 2006-07-11
12:29:15) [
Session ID: a129e22b0d979a5e287c2bdf3c662916
Parameters: {“action”=>“login”, “username”=>“Lunenburg”,
“controller”=>“account”, “password”=>“XXXXX”}
^[[4;35;1mPreference Columns (0.000231)^[[0m ^[[0mSHOW FIELDS
FROM preferences^[[0m
^[[4;36;1mPreference Load (0.000177)^[[0m ^[[0;1mSELECT * FROM
preferences W
HERE (username = ‘Lunenburg’) LIMIT 1^[[0m
DEBUG: Setting session pref to #Preference:0x8f43e38
DEBUG: Set session pref to #Preference:0x8f43e38
Redirected to http://carlton.skiltech.com:3000/forum/jump/0
Completed in 3.25367 (0 reqs/sec) | DB: 0.00041 (0%) | 302 Found
You have a nil object when you didn’t expect it!
The error occurred while evaluating nil.sweep^M
roller/flash.rb:144:in `process_cleanup_without_filters’^M

Thinking it was some caching issue, I set
ActionController::Base.perform_caching = false in my environment.rb,
with no luck.

I realize it’s kind of vague, but does anyone have any suggestions on
where to look to find out why I’m getting a “nil.sweep” in my code?