diff --git a/lib/jsonapi/active_model_error_serializer.rb b/lib/jsonapi/active_model_error_serializer.rb
index 59a3e3c..3968142 100644
--- a/lib/jsonapi/active_model_error_serializer.rb
+++ b/lib/jsonapi/active_model_error_serializer.rb
@@ -13,7 +13,9 @@ class ActiveModelErrorSerializer < ErrorSerializer
 
     attribute :code do |object|
       _, error_hash = object
-      code = error_hash[:error] unless error_hash[:error].is_a?(Hash)
+      unless error_hash[:error].is_a?(Hash) || error_hash[:error].is_a?(String)
+        code = error_hash[:error]
+      end
       code ||= error_hash[:message] || :invalid
       # `parameterize` separator arguments are different on Rails 4 vs 5...
       code.to_s.delete("''").parameterize.tr('-', '_')
@@ -29,9 +31,15 @@ class ActiveModelErrorSerializer < ErrorSerializer
           error_key, nil, error_hash[:error]
         )
       elsif error_hash[:error].present?
-        message = errors_object.generate_message(
-          error_key, error_hash[:error], error_hash
-        )
+        # if the error was added as a string, we do not need to generate an
+        # error message, but use the error as is
+        if error_hash[:error].is_a?(String)
+          message = error_hash[:error]
+        else
+          message = errors_object.generate_message(
+            error_key, error_hash[:error], error_hash
+          )
+        end
       else
         message = error_hash[:message]
       end
diff --git a/spec/dummy.rb b/spec/dummy.rb
index d273f81..eca55f0 100644
--- a/spec/dummy.rb
+++ b/spec/dummy.rb
@@ -43,6 +43,21 @@ class Note < ActiveRecord::Base
   validates_format_of :title, without: /BAD_TITLE/
   validates_numericality_of :quantity, less_than: 100, if: :quantity?
   belongs_to :user, required: true
+
+  validate :nice_title
+
+  # this validation is added to test that a validation error added as a string
+  # (instead of a symbol) works as well
+  #
+  # see https://api.rubyonrails.org/v7.0.8.7/classes/ActiveModel/Errors.html#method-i-add
+  #
+  # > If `type` is a string, it will be used as error message.
+  #
+  def nice_title
+    return unless title == 'NOT_SO_NICE_TITLE'
+
+    errors.add(:title, 'is not so nice')
+  end
 end
 
 class CustomNoteSerializer
diff --git a/spec/errors_spec.rb b/spec/errors_spec.rb
index 38cf978..517071b 100644
--- a/spec/errors_spec.rb
+++ b/spec/errors_spec.rb
@@ -119,6 +119,27 @@
         end
       end
 
+      context 'with a validation error type that is a string' do
+        let(:params) do
+          payload = note_params.dup
+          payload[:data][:attributes][:title] = 'NOT_SO_NICE_TITLE'
+          payload
+        end
+
+        it do
+          expect(response).to have_http_status(:unprocessable_entity)
+          expect(response_json['errors'].size).to eq(2)
+          expect(response_json['errors'][0]['status']).to eq('422')
+          expect(response_json['errors'][0]['code']).to include('invalid')
+          expect(response_json['errors'][0]['title'])
+            .to eq(Rack::Utils::HTTP_STATUS_CODES[422])
+          expect(response_json['errors'][0]['source'])
+              .to eq('pointer' => '/data/attributes/title')
+          expect(response_json['errors'][0]['detail'])
+            .to eq('Title is not so nice')
+        end
+      end
+
       context 'as a param attribute' do
         let(:params) do
           payload = note_params.dup
diff --git a/spec/pagination_spec.rb b/spec/pagination_spec.rb
index ec1c3c4..bdc5fcc 100644
--- a/spec/pagination_spec.rb
+++ b/spec/pagination_spec.rb
@@ -53,6 +53,8 @@
 
         context 'on page 2 out of 3' do
           let(:as_list) { }
+          # .compact_blank is not available in our Rails version
+          # rubocop:disable Rails/CompactBlank
           let(:params) do
             {
               page: { number: 2, size: 1 },
@@ -60,6 +62,7 @@
               as_list: as_list
             }.reject { |_k, _v| _v.blank? }
           end
+          # rubocop:enable Rails/CompactBlank
 
           context 'on an array of resources' do
             let(:as_list) { true }
@@ -153,12 +156,15 @@
 
         context 'on paging beyond the last page' do
           let(:as_list) { }
+          # .compact_blank is not available in our Rails version
+          # rubocop:disable Rails/CompactBlank
           let(:params) do
             {
               page: { number: 5, size: 1 },
               as_list: as_list
             }.reject { |_k, _v| _v.blank? }
           end
+          # rubocop:enable Rails/CompactBlank
 
           context 'on an array of resources' do
             let(:as_list) { true }