Can't get uniqueness validation test pass with shoulda matcher

I have a shoulda matcher in my avatar_parts_spec.rb and I can't get it pass:

Test:

require 'rails_helper'

RSpec.describe AvatarPart, :type => :model do
  it { should validate_presence_of(:name) }
  it { should validate_presence_of(:type) }
  it { should validate_uniqueness_of(:name).case_insensitive }
  it { should belong_to(:avatar) }
end

Model:

class AvatarPart < ActiveRecord::Base
  attr_accessible :name, :type, :avatar_id

  belongs_to :avatar

  validates_uniqueness_of :name, case_sensitive: false
  validates :name, :type, presence: true, allow_blank: false
end

Migration:

class CreateAvatarParts < ActiveRecord::Migration
  def change
    create_table :avatar_parts do |t|
      t.string :name, null: false
      t.string :type, null: false
      t.integer :avatar_id      

      t.timestamps
    end
  end
end

Error:

 1) AvatarPart should require unique value for name
     Failure/Error: it { should validate_uniqueness_of(:name).case_insensitive }
     ActiveRecord::StatementInvalid:
       SQLite3::ConstraintException: NOT NULL constraint failed: avatar_parts.type: INSERT INTO "avatar_parts" ("avatar_id", "created_at", "name", "type", "updated_at") VALUES (?, ?, ?, ?, ?)

What could be the cause of the error?

Edit: Github repo: https://github.com/preciz/avatar_parts

The documentation for that matcher says:

This matcher works a bit differently than other matchers. As noted before, it will create an instance of your model if one doesn't already exist. Sometimes this step fails, especially if you have database-level restrictions on any attributes other than the one which is unique. In this case, the solution is to populate these attributes with before you call validate_uniqueness_of.

So in your case the solution would be something like:

  describe "uniqueness" do
    subject { AvatarPart.new(name: "something", type: "something else") }
    it { should validate_uniqueness_of(:name).case_insensitive }
  end