diff --git a/app/models/account.rb b/app/models/account.rb
index 9c6a607..c185c22 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -39,7 +39,9 @@ class Account < ApplicationRecord
 
   validates :public_name, allow_nil: true, length: { in: 1..255 }
 
-  validates :biography, allow_nil: true, length: { in: 1..10_000 }
+  validates :biography,
+            allow_nil: true,
+            good_big_text: true
 
   validates :avatar, allow_nil: true, image: true
 
diff --git a/app/models/session.rb b/app/models/session.rb
index a0face9..1b5ba86 100644
--- a/app/models/session.rb
+++ b/app/models/session.rb
@@ -22,12 +22,14 @@ class Session < ApplicationRecord
 
   validates :ip_address, presence: true
 
-  validates :user_agent, length: { maximum: 10_000 }
+  validates :user_agent,
+            allow_nil: true,
+            good_big_text: true
 
 private
 
   def turn_nils_into_blanks
-    self.user_agent = '' if user_agent.blank?
+    self.user_agent = nil if user_agent.blank?
   end
 
   def strip_extra_spaces
diff --git a/app/validators/good_big_text_validator.rb b/app/validators/good_big_text_validator.rb
new file mode 100644
index 0000000..36b54a5
--- /dev/null
+++ b/app/validators/good_big_text_validator.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class GoodBigTextValidator < GoodTextValidator
+  class Validation < Validation
+    MIN = 1
+    MAX = 10_000
+
+    def perform
+      super
+      error! :too_short, count: MIN if value.to_s.length < MIN
+      error! :too_long,  count: MAX if value.to_s.length > MAX
+    end
+  end
+end
diff --git a/db/migrate/20190929131544_change_user_agent.rb b/db/migrate/20190929131544_change_user_agent.rb
new file mode 100644
index 0000000..bc17b33
--- /dev/null
+++ b/db/migrate/20190929131544_change_user_agent.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class ChangeUserAgent < ActiveRecord::Migration[6.0]
+  include Partynest::Migration
+
+  def change
+    drop_constraint :sessions, :user_agent, <<~SQL
+      user_agent = '' OR is_good_big_text(user_agent)
+    SQL
+
+    remove_column :sessions, :user_agent, :string, null: false, default: ''
+    add_column    :sessions, :user_agent, :string, null: true
+
+    constraint :sessions, :user_agent, <<~SQL
+      user_agent IS NULL OR is_good_big_text(user_agent)
+    SQL
+  end
+end
diff --git a/db/structure.sql b/db/structure.sql
index 114e503..8605418 100644
--- a/db/structure.sql
+++ b/db/structure.sql
@@ -763,9 +763,9 @@ CREATE TABLE public.sessions (
     account_id bigint NOT NULL,
     logged_at timestamp without time zone NOT NULL,
     ip_address character varying NOT NULL,
-    user_agent character varying DEFAULT ''::character varying NOT NULL,
+    user_agent character varying,
     CONSTRAINT ip_address CHECK (public.is_good_small_text((ip_address)::text)),
-    CONSTRAINT user_agent CHECK ((((user_agent)::text = ''::text) OR public.is_good_big_text((user_agent)::text)))
+    CONSTRAINT user_agent CHECK (((user_agent IS NULL) OR public.is_good_big_text((user_agent)::text)))
 );
 
 
@@ -1617,6 +1617,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20190921142404'),
 ('20190921161613'),
 ('20190921191213'),
-('20190928171705');
+('20190928171705'),
+('20190929131544');
 
 
diff --git a/factories/sessions.rb b/factories/sessions.rb
index 9aa09d7..ac73afd 100644
--- a/factories/sessions.rb
+++ b/factories/sessions.rb
@@ -13,6 +13,10 @@ FactoryBot.define do
     trait :with_ipv6_address do
       ip_address { Faker::Internet.ip_v6_address }
     end
+
+    trait :without_user_agent do
+      user_agent { nil }
+    end
   end
 
   factory :some_session_with_ipv6_address,
diff --git a/spec/models/session_spec.rb b/spec/models/session_spec.rb
index 9df54bc..6974e37 100644
--- a/spec/models/session_spec.rb
+++ b/spec/models/session_spec.rb
@@ -33,19 +33,19 @@ RSpec.describe Session do
 
     it { is_expected.to validate_length_of(:user_agent).is_at_most(10_000) }
 
-    it { is_expected.to allow_value '' }
+    it { is_expected.to allow_value nil }
     it { is_expected.to allow_value Faker::Internet.user_agent }
 
     context 'when it was set to nil' do
       subject { build :some_session, user_agent: nil }
       before { subject.validate }
-      specify { expect(subject.user_agent).to eq '' }
+      specify { expect(subject.user_agent).to eq nil }
     end
 
     context 'when it was set to blank' do
       subject { build :some_session, user_agent: ' ' * rand(1..3) }
       before { subject.validate }
-      specify { expect(subject.user_agent).to eq '' }
+      specify { expect(subject.user_agent).to eq nil }
     end
 
     context 'when it has leading spaces' do
diff --git a/spec/validators/good_big_text_validator_spec.rb b/spec/validators/good_big_text_validator_spec.rb
new file mode 100644
index 0000000..0df863d
--- /dev/null
+++ b/spec/validators/good_big_text_validator_spec.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe GoodBigTextValidator do
+  pending "add some examples to (or delete) #{__FILE__}"
+end