NoMethodError (undefined method `to_sym' for {"6"=>"1"}:Hash

I've got a search form that was working on rails 1.2.6, but since
upgrading to 2.0.2, I'm getting this error when I post my form. Here's
the full error:

Processing SearchController#results (for 24.173.26.226 at 2008-01-24
14:51:23) [GET]
  Session ID:
BAh7CDoTc2hvd1RodW1ibmFpbHMiBjE6CXVzZXJvOglVc2VyBzoWQGF0dHJp%0AYnV0ZXNfY2FjaGV7ADoQQGF0dHJpYnV0ZXN7DCIJbmFtZSITRG91ZyBKb2hu%0Ac3RvbmUiDWNhdGVnb3J5Ig5zdXBlcnVzZXIiB2lkIgYyIg1wYXNzd29yZCIN%0AcEBzc3dvcmQiCmVtYWlsIiNkam9obnN0b25lQHByb3ZpbmNldG93bi1tYS5n%0Ab3YiC2FjdGl2ZSIGMSIPY3JlYXRlZF9hdCIYMjAwOC0wMS0yMyAxMzozNToz%0AOCIKZmxhc2hJQzonQWN0aW9uQ29udHJvbGxlcjo6Rmxhc2g6OkZsYXNoSGFz%0AaHsABjoKQHVzZWR7AA%3D%3D--ec73422aae883d2a819f7c304ecc59ed347e3337
  Parameters: {"format"=>{"6"=>"1"}, "collection"=>{"5"=>"1"},
"search"=>{"form"=>"advancedsearch", "keyword"=>"oil"}, "x"=>"17",
"y"=>"13", "action"=>"results", "controller"=>"search"}

NoMethodError (undefined method `to_sym' for
{"6"=>"1"}:HashWithIndifferentAccess):
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:420:in
`template_format'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_view/base.rb:396:in
`pick_template_extension'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1259:in
`template_exempt_from_layout?'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/layout.rb:278:in
`apply_layout?'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/layout.rb:256:in
`render_without_benchmark'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:51:in
`render'
    /usr/lib/ruby/1.8/benchmark.rb:293:in `measure'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:51:in
`render'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1153:in
`default_render'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:1159:in
`perform_action_without_filters'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:697:in
`call_filters'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:689:in
`perform_action_without_benchmark'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:68:in
`perform_action_without_rescue'
    /usr/lib/ruby/1.8/benchmark.rb:293:in `measure'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/benchmarking.rb:68:in
`perform_action_without_rescue'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/rescue.rb:199:in
`perform_action_without_caching'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/caching.rb:678:in
`perform_action'
    /usr/lib64/ruby/gems/1.8/gems/activerecord-2.0.2/lib/active_record/connection_adapters/abstract/query_cache.rb:33:in
`cache'
    /usr/lib64/ruby/gems/1.8/gems/activerecord-2.0.2/lib/active_record/query_cache.rb:8:in
`cache'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/caching.rb:677:in
`perform_action'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:524:in
`send'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:524:in
`process_without_filters'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/filters.rb:685:in
`process_without_session_management_support'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/session_management.rb:123:in
`process'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/base.rb:388:in
`process'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:171:in
`handle_request'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:115:in
`dispatch'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:126:in
`dispatch_cgi'
    /usr/lib64/ruby/gems/1.8/gems/actionpack-2.0.2/lib/action_controller/dispatcher.rb:9:in
`dispatch'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/rails.rb:76:in
`process'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/rails.rb:74:in
`synchronize'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/rails.rb:74:in
`process'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:159:in
`process_client'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:158:in
`each'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:158:in
`process_client'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:285:in
`run'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:285:in
`initialize'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:285:in
`new'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:285:in
`run'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:268:in
`initialize'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:268:in
`new'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel.rb:268:in
`run'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/configurator.rb:282:in
`run'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/configurator.rb:281:in
`each'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/configurator.rb:281:in
`run'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/mongrel_rails:128:in
`run'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/../lib/mongrel/command.rb:212:in
`run'
    /usr/lib64/ruby/gems/1.8/gems/mongrel-1.1.3/bin/mongrel_rails:281
    /usr/bin/mongrel_rails:19:in `load'
    /usr/bin/mongrel_rails:19