After migrating to Bundler: ActionView::TemplateError (private method `gsub' called for nil:NilClass)

I configured bundler for a Rails 2 app as I have done other times, but
this time I get:

Processing ApplicationController#index (for 127.0.0.1 at 2011-03-22
14:17:47) [GET]

ActionView::TemplateError (private method `gsub' called for
nil:NilClass) on line #7 of rails/ruby/1.8/gems/actionpack-2.3.11/lib/
action_controller/templates/rescues/diagnostics.erb:
4: in <%=h request.parameters['controller'].humanize %>Controller<
% if request.parameters['action'] %>#<%=h request.parameters['action']
%><% end %>
5: <% end %>
6: </h1>
7: <pre><%=h @exception.clean_message %></pre>
8:
9: <%= render :file => @rescues_path["rescues/_trace.erb"] %>
10:

    rails/ruby/1.8/gems/activesupport-2.3.11/lib/active_support/
whiny_nil.rb:52:in `method_missing'
    rails/ruby/1.8/gems/activesupport-2.3.11/lib/active_support/
core_ext/pathname/clean_within.rb:7:in `clean_within'
    rails/ruby/1.8/gems/activesupport-2.3.11/lib/active_support/
core_ext/exception.rb:12:in `clean_message'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
templates/rescues/diagnostics.erb:7
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_view/
renderable.rb:34:in `send'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_view/
renderable.rb:34:in `render'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_view/base.rb:
306:in `with_template'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_view/
renderable.rb:30:in `render'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_view/template.rb:
205:in `render_template'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_view/base.rb:
265:in `render'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
rescue.rb:134:in `rescue_action_locally'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
rescue.rb:152:in `rescue_action_without_handler'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
rescue.rb:74:in `rescue_action'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
rescue.rb:162:in `perform_action_without_flash'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
flash.rb:151:in `perform_action'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
base.rb:532:in `send'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
base.rb:532:in `process_without_filters'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
filters.rb:606:in `process'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
base.rb:391:in `process'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
base.rb:386:in `call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
routing/route_set.rb:438:in `call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
dispatcher.rb:87:in `dispatch'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
dispatcher.rb:121:in `_call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
dispatcher.rb:130:in `build_middleware_stack'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
string_coercion.rb:25:in `call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
string_coercion.rb:25:in `call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
params_parser.rb:15:in `call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
session/abstract_store.rb:177:in `call'
    rails/ruby/1.8/gems/activerecord-2.3.11/lib/active_record/
query_cache.rb:29:in `call'
    rails/ruby/1.8/gems/activerecord-2.3.11/lib/active_record/
connection_adapters/abstract/query_cache.rb:34:in `cache'
    rails/ruby/1.8/gems/activerecord-2.3.11/lib/active_record/
query_cache.rb:9:in `cache'
    rails/ruby/1.8/gems/activerecord-2.3.11/lib/active_record/
query_cache.rb:28:in `call'
    rails/ruby/1.8/gems/activerecord-2.3.11/lib/active_record/
connection_adapters/abstract/connection_pool.rb:361:in `call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
failsafe.rb:26:in `call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
dispatcher.rb:114:in `call'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
reloader.rb:34:in `run'
    rails/ruby/1.8/gems/actionpack-2.3.11/lib/action_controller/
dispatcher.rb:108:in `call'
    rails/ruby/1.8/gems/rails-2.3.11/lib/rails/rack/static.rb:31:in
`call'
    rails/ruby/1.8/gems/rails-2.3.11/lib/rails/rack/log_tailer.rb:
17:in `call'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/httpserver.rb:104:in `service'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/httpserver.rb:65:in `run'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/server.rb:173:in `start_thread'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/server.rb:162:in `start'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/server.rb:162:in `start_thread'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/server.rb:95:in `start'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/server.rb:92:in `each'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/server.rb:92:in `start'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/server.rb:23:in `start'
    /Users/riccardotacconi/.rvm/rubies/ree-1.8.7-2011.03/lib/ruby/1.8/
webrick/server.rb:82:in `start'
    rails/ruby/1.8/gems/rails-2.3.11/lib/commands/server.rb:111

I configured bundler for a Rails 2 app as I have done other times, but
this time I get:

Processing ApplicationController#index (for 127.0.0.1 at 2011-03-22
14:17:47) [GET]

ActionView::TemplateError (private method `gsub' called for
nil:NilClass) on line #7 of rails/ruby/1.8/gems/actionpack-2.3.11/lib/
action_controller/templates/rescues/diagnostics.erb:
4: in <%=h request.parameters['controller'].humanize %>Controller<
% if request.parameters['action'] %>#<%=h request.parameters['action']
%><% end %>
5: <% end %>
6: </h1>
7: <pre><%=h @exception.clean_message %></pre>

Is @exception nil possibly?

Colin