Using array in Rails JSONB column

I want to store an array of objects in JSONB column in PostgreSQL. I'm using Rails 5.2. I'm using a custom serializer that ensures the value assigned to a JSONB field is an Array not a Hash. And I'm getting an error when assigning something like [{a: 1}] to the field. Here is the code:
model:

class Printing
  serialize :card_faces, CardFacesSerializer
end

serializer:

class CardFacesSerializer
  include JSONBArraySerializer

  def allowed_attributes
    %i[name image]
  end
end

serializer concern:

module JSONBArraySerializer
  extend ActiveSupport::Concern

  def initialize(data)
    return [] if data.blank?

    if data.is_a?(String)
      json = Oj.load(data, symbol_keys: true)
    end
    raise ArgumentError, "#{json} must be [{},{}], not {}" if json.is_a?(Hash)
    # Will only set the properties that are allowed
    json.map do |hash|
      hash.slice(self.allowed_attributes)
    end
  end

  class_methods do
    def load(json)
      return [] if json.blank?
      self.new(json)
    end

    def dump(obj)
      # Make sure the type is right.
      if obj.is_a?(self)
        obj.to_json
      else
       raise StandardError, "Expected #{self}, got #{obj.class}"
      end
    end
  end
end

When evaluating:

pr = Printing.first
pr.card_faces = [{hay: 12}]
pr.save!

I get an error:

StandardError: Expected CardFacesSerializer, got Array

I don't think it's clear for me how dump/load work. Why dump is being called during save? How can I fix my code to work properly?

UPDATE
I managed to make it work with this code of serializer concern:

module JSONBArraySerializer
  extend ActiveSupport::Concern

  class_methods do
    def load(data)
      return [] if data.blank?

      if data.is_a?(String)
        json = Oj.load(data, symbol_keys: true)
      end
      raise ArgumentError, "#{json} must be [{},{}], not {}" if json.is_a?(Hash)

      # Will only set the properties that are allowed
      json.map do |hash|
        hash.slice(*allowed_attributes)
      end
    end

    def dump(obj)
      # Make sure the type is right.
      if obj.is_a?(Array)
        obj.to_json
      else
       raise ArgumentError, "Expected Array, got #{obj.class}"
      end
    end
  end
end