diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3ee83edaaf..9c1c75e85d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 ### Bug fixes
 
 * [#345](https://github.com/rubocop-hq/rubocop-rails/issues/345): Fix error of `Rails/AfterCommitOverride` on `after_commit` with a lambda. ([@pocke][])
+* [#349](https://github.com/rubocop-hq/rubocop-rails/pull/349): Fix errors of `Rails/UniqueValidationWithoutIndex`. ([@Tietew][])
 
 ## 2.8.0 (2020-09-04)
 
@@ -281,3 +282,4 @@
 [@mobilutz]: https://github.com/mobilutz
 [@bubaflub]: https://github.com/bubaflub
 [@dvandersluis]: https://github.com/dvandersluis
+[@Tietew]: https://github.com/Tietew
diff --git a/lib/rubocop/cop/mixin/active_record_helper.rb b/lib/rubocop/cop/mixin/active_record_helper.rb
index 8021d60028..2611c068d3 100644
--- a/lib/rubocop/cop/mixin/active_record_helper.rb
+++ b/lib/rubocop/cop/mixin/active_record_helper.rb
@@ -52,6 +52,7 @@ def table_name(class_node)
       # @param table [RuboCop::Rails::SchemaLoader::Table]
       # @return [String, nil]
       def resolve_relation_into_column(name:, class_node:, table:)
+        return unless table
         return name if table.with_column?(name: name)
 
         find_belongs_to(class_node) do |belongs_to|
diff --git a/lib/rubocop/cop/rails/unique_validation_without_index.rb b/lib/rubocop/cop/rails/unique_validation_without_index.rb
index 35c362b978..06cdd8d0f3 100644
--- a/lib/rubocop/cop/rails/unique_validation_without_index.rb
+++ b/lib/rubocop/cop/rails/unique_validation_without_index.rb
@@ -46,6 +46,8 @@ def on_send(node)
 
         def find_schema_information(node)
           klass = class_node(node)
+          return unless klass
+
           table = schema.table_by(name: table_name(klass))
           names = column_names(node)
 
diff --git a/spec/rubocop/cop/rails/unique_validation_without_index_spec.rb b/spec/rubocop/cop/rails/unique_validation_without_index_spec.rb
index 0cb7200fdd..491e3da7b7 100644
--- a/spec/rubocop/cop/rails/unique_validation_without_index_spec.rb
+++ b/spec/rubocop/cop/rails/unique_validation_without_index_spec.rb
@@ -495,5 +495,44 @@ class User
         end
       end
     end
+
+    context 'when the table does not exist' do
+      let(:schema) { <<~RUBY }
+        ActiveRecord::Schema.define(version: 2020_02_02_075409) do
+          create_table "users", force: :cascade do |t|
+            t.string "account", null: false, unique: true
+          end
+        end
+      RUBY
+
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          class Article
+            validates :account, uniqueness: true
+          end
+        RUBY
+      end
+    end
+
+    context 'when module' do
+      let(:schema) { <<~RUBY }
+        ActiveRecord::Schema.define(version: 2020_02_02_075409) do
+          create_table "users", force: :cascade do |t|
+            t.string "account", null: false, unique: true
+          end
+        end
+      RUBY
+
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          module User
+            extend ActiveSupport::Concern
+            included do
+              validates :account, uniqueness: true
+            end
+          end
+        RUBY
+      end
+    end
   end
 end