undefined method `in_time_zone' for nil:NilClass for sensor 2, but not for sensor 1

[rhelfter]

Processing PagesController#events_for_sensor (for ::ffff:192.168.5.136 at 2010-07-20 16:44:13) [GET]
Parameters: {"sensor"=>"2", "controller"=>"pages", "action"=>"events_for_sensor"}
Rendering template within layouts/application
Rendering pages/events_for_sensor

ActionView::TemplateError (undefined method `in_time_zone' for nil:NilClass) on line #5 of app/views/events/_event.html.erb:
2:

3:
4:

5: <%= time_ago_in_words(Chronic.parse(event.timestamp.in_time_zone('UTC'))) %> ago - <%= event.timestamp.getutc.strftime('%D - %I:%M:%S %p') %>

6:

7:
8:

app/views/events/_event.html.erb:5
app/views/pages/events_for_sensor.html.erb:4
<internal:prelude>:8:in `synchronize'
/apps/IPsoft/ruby191/lib/ruby/1.9.1/webrick/httpserver.rb:111:in `service'
/apps/IPsoft/ruby191/lib/ruby/1.9.1/webrick/httpserver.rb:70:in `run'
/apps/IPsoft/ruby191/lib/ruby/1.9.1/webrick/server.rb:183:in `block in start_thread'

Rendering template within layouts/application
Rendering /errors/500.html.erb (500 Internal Server Error)