Please help(writing conditions using haml)

in ma project there is a model named campaigns and delivery_schedules .
in delivery_schedules i defined a method called below

def get_scheduled_time

self.delivery_date.to_s << " " << self.delivery_hour.to_s << “:” <<
self.delivery_minute.to_s << " " << self.delivery_meridiem.upcase
in ma view page i write code lik this to print those values like this

%b #{t :delivery_status}:
= t(campaign.delivery_status,:On_Date,:time_spec

but i am gettting errors lik this

Showing app/views/campaigns/_metrics.html.haml where line #19 raised:

You have a nil object when you didn’t expect it!
The error occurred while evaluating nil.delivery_schedule

also in a yml file i defined some parameters lik this

delivery_status: Delivery Status
on: on
On_Date: “On {{time_spec}}”
this is the result i am expecting

Scheduled On Mar 30, 2011 07:00 ( Scheduled On{{ value}}

can anyone pls help me or giv me idea to solve it