diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 14906e441b56c..d389d7f6fe0ca 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Fixed an error which would occur in dirty checking when calling
+    `update_attributes` from a getter.
+
+    Fixes #20531.
+
+    *Sean Griffin*
+
 *   Make `remove_foreign_key` reversible. Any foreign key options must be
     specified, similar to `remove_column`.
 
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 7ba907f78649a..0171ef3bdfbed 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -108,6 +108,7 @@ def raw_write_attribute(attr, value)
       end
 
       def save_changed_attribute(attr, old_value)
+        clear_changed_attributes_cache
         if attribute_changed_by_setter?(attr)
           clear_attribute_changes(attr) unless _field_changed?(attr, old_value)
         else
@@ -176,7 +177,11 @@ def cache_changed_attributes
         @cached_changed_attributes = changed_attributes
         yield
       ensure
-        remove_instance_variable(:@cached_changed_attributes)
+        clear_changed_attributes_cache
+      end
+
+      def clear_changed_attributes_cache
+        remove_instance_variable(:@cached_changed_attributes) if defined?(@cached_changed_attributes)
       end
     end
   end
diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index 216f228142e07..f5aaf22e13ef8 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -703,6 +703,22 @@ def test_datetime_attribute_doesnt_change_if_zone_is_modified_in_string
     assert pirate.catchphrase_changed?(from: "arrrr", to: "arrrr matey!")
   end
 
+  test "getters with side effects are allowed" do
+    klass = Class.new(Pirate) do
+      def catchphrase
+        if super.blank?
+          update_attribute(:catchphrase, "arr") # what could possibly go wrong?
+        end
+        super
+      end
+    end
+
+    pirate = klass.create!(catchphrase: "lol")
+    pirate.update_attribute(:catchphrase, nil)
+
+    assert_equal "arr", pirate.catchphrase
+  end
+
   private
     def with_partial_writes(klass, on = true)
       old = klass.partial_writes?