[Rails] undefined method '<<' for polymorphic association

[fugee ohu]

class Item < ApplicationRecord
 has_many :pictures, as: :imageable
end

class Picture < ApplicationRecord
 belongs_to :imageable, polymorphic: true, optional: true
end

NoMethodError in PicturesController#create undefined method `<<' for 
#<Picture:0x007f3758cc88f0>

@item.pictures.create << @picture

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