[Rails] NoMethodError in XXXXXXXXXController#create

Hello comrades ,

I got a error. I don't understand that exception. What happened ?

You have a nil object when you didn't expect it!
You might have expected an instance of ActiveRecord::Base.
The error occurred while evaluating nil.[]

i written this code:
if @personal_information.save
HamtranZeeldegchs.new(params[:hamtran_zeeldegch]['0']) # This line
is may a error.
hzs = HamtranZeeldegchs.new(params[:hamtran_zeeldegch]['0'])
hzs.personal_information_id = @personal_information.id
hzs.save

MODEL:

class PersonalInformation < ActiveRecord::Base
has_many :hamtran_zeeldegchs,:foreign_key => 'personal_information_id' ,
:class_name => 'HamtranZeeldegchs', :dependent => :destroy

class HamtranZeeldegchs < ActiveRecord::Base
belongs_to :personal_information , :class_name => 'PersonalInformation',
:foreign_key => ':personal_information_id'

Can anyone help me ?

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Thanks
Tsolmon