diff --git a/activemodel/lib/active_model/attribute_mutation_tracker.rb b/activemodel/lib/active_model/attribute_mutation_tracker.rb
index 82d90664c0..6abf37bd44 100644
--- a/activemodel/lib/active_model/attribute_mutation_tracker.rb
+++ b/activemodel/lib/active_model/attribute_mutation_tracker.rb
@@ -27,7 +27,7 @@ module ActiveModel
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
         change = change_to_attribute(attr_name)
         if change
-          result[attr_name] = change
+          result.merge!(attr_name => change)
         end
       end
     end
diff --git a/activerecord/test/cases/dirty_test.rb b/activerecord/test/cases/dirty_test.rb
index 2f1434b2bc..5c7f70b7a0 100644
--- a/activerecord/test/cases/dirty_test.rb
+++ b/activerecord/test/cases/dirty_test.rb
@@ -473,6 +473,14 @@ class DirtyTest < ActiveRecord::TestCase
     end
   end
 
+  def test_changes_to_save_should_not_mutate_array_of_hashes
+    topic = Topic.new(author_name: "Bill", content: [{ a: "a" }])
+
+    topic.changes_to_save
+
+    assert_equal [{ a: "a" }], topic.content
+  end
+
   def test_previous_changes
     # original values should be in previous_changes
     pirate = Pirate.new