Ruby on Rails

I am using cocoon from github to deal with accepts_nested_attributes_for. In my partial i am trying to manipulate nested attributes but it is not working. My code is in my partial :

.nested-fields
  = f.inputs do
    = f.object.myarrayfield.length

I have an error message :

undefined method `length' for nil:NilClass

If I do only that

.nested-fields
  = f.inputs do
    = f.object.myarrayfield

There is no error message and the page give me the value of the field :

[1,2]

The length method should give me 2 not nil, do you know why ?

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