NoMethodError (undefined method `call' for nil:NilClass):

I am getting HTTP 500 Internal server error when updating info for one
of page of my website. Please help me and for your reference i have
posted ruby trace below:

Processing ModelsController#update (for 203.187.233.94 at 2009-03-20
02:40:06) [PUT]
  Session ID:
BAh7CDoMY3NyZl9pZCIlYTc0NDc5OWI1ZDNhMDM4YmVjZTY0YjE3NGZkODQ2
YjAiCmZsYXNoSUM6J0FjdGlvbkNvbnRyb2xsZXI6OkZsYXNoOjpGbGFzaEhh
c2h7AAY6CkB1c2VkewA6DHVzZXJfaWRpBw==--0cc7b78313f817b2be3cc19a19164dbfa9a351e9
  Parameters: {"commit"=>"Update", "action"=>"update",
"_method"=>"put",
"authenticity_token"=>"47bc4f13abf32c82e7602e1ebd52f5a5338a566d",
"id"=>"12", "controller"=>"admin/models", "model"=>{"city"=>"Surat",
"biography"=>"", "measurements"=>"", "weight"=>"110",
"agency_rep_email"=>"", "agency_address"=>"", "country_id"=>"1",
"approved"=>"0", "agency_name"=>"", "region_id"=>"1", "sex"=>"m",
"agency_rep_name"=>"", "height"=>"5'3", "first_name"=>"Jignesh",
"agency_phone"=>"", "eye_color_id"=>"3", "hair_color_id"=>"9",
"age"=>"30", "last_name"=>"Wadia"}}

NoMethodError (undefined method `call' for nil:NilClass):
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/mime_responds.rb:156:in `respond'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/mime_responds.rb:154:in `each'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/mime_responds.rb:154:in `respond'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/mime_responds.rb:107:in `respond_to'
    /app/controllers/admin/models_controller.rb:64:in `update'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/base.rb:1162:in `send'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/base.rb:1162:in `perform_action_without_filters'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/filters.rb:580:in `call_filters'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/filters.rb:573:in `perform_action_without_benchmark'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/benchmarking.rb:68:in
`perform_action_without_rescue'
    /usr/local/lib/ruby/1.8/benchmark.rb:293:in `measure'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/benchmarking.rb:68:in
`perform_action_without_rescue'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/rescue.rb:201:in `perform_action_without_caching'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/caching/sql_cache.rb:13:in `perform_action'
    /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/
active_record/connection_adapters/abstract/query_cache.rb:33:in
`cache'
    /usr/local/lib/ruby/gems/1.8/gems/activerecord-2.1.0/lib/
active_record/query_cache.rb:8:in `cache'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/caching/sql_cache.rb:12:in `perform_action'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/base.rb:529:in `send'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/base.rb:529:in `process_without_filters'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/filters.rb:569:in
`process_without_session_management_support'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/session_management.rb:130:in `process'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/base.rb:389:in `process'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/dispatcher.rb:149:in `handle_request'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/dispatcher.rb:107:in `dispatch'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/dispatcher.rb:104:in `synchronize'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/dispatcher.rb:104:in `dispatch'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/dispatcher.rb:120:in `dispatch_cgi'
    /usr/local/lib/ruby/gems/1.8/gems/actionpack-2.1.0/lib/
action_controller/dispatcher.rb:35:in `dispatch'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
103:in `process_request'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
153:in `with_signal_handler'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
101:in `process_request'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
78:in `process_each_request'
    /usr/local/lib/ruby/gems/1.8/gems/fcgi-0.8.7/lib/fcgi.rb:612:in
`each_cgi'
    /usr/local/lib/ruby/gems/1.8/gems/fcgi-0.8.7/lib/fcgi.rb:609:in
`each'
    /usr/local/lib/ruby/gems/1.8/gems/fcgi-0.8.7/lib/fcgi.rb:609:in
`each_cgi'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
77:in `process_each_request'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
76:in `catch'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
76:in `process_each_request'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
50:in `process!'
    /usr/local/lib/ruby/gems/1.8/gems/rails-2.1.0/lib/fcgi_handler.rb:
24:in `process!'
    dispatch.fcgi:24

Rendering /home/canadian/canadianmodelscouts.com/public/500.html (500
Internal Server Error)

REgs,
Jignesh Wadia

it would be a good idea to post the controller method update and the
class Model.

in this branch you're not telling rails what to do - you've commented
out all your format.bla calls

Fred