diff --git a/activemodel/lib/active_model/attribute_assignment.rb b/activemodel/lib/active_model/attribute_assignment.rb
index 9bdec0dfda..ea4dae101c 100644
--- a/activemodel/lib/active_model/attribute_assignment.rb
+++ b/activemodel/lib/active_model/attribute_assignment.rb
@@ -26,13 +26,12 @@ module ActiveModel
     #   cat.name # => 'Gorby'
     #   cat.status # => 'sleeping'
     def assign_attributes(new_attributes)
-      if !new_attributes.respond_to?(:stringify_keys)
+      unless new_attributes.respond_to?(:each_pair)
         raise ArgumentError, "When assigning attributes, you must pass a hash as an argument, #{new_attributes.class} passed."
       end
       return if new_attributes.empty?
 
-      attributes = new_attributes.stringify_keys
-      _assign_attributes(sanitize_for_mass_assignment(attributes))
+      _assign_attributes(sanitize_for_mass_assignment(new_attributes))
     end
 
     alias attributes= assign_attributes
@@ -49,7 +48,7 @@ module ActiveModel
         if respond_to?(setter)
           public_send(setter, v)
         else
-          raise UnknownAttributeError.new(self, k)
+          raise UnknownAttributeError.new(self, k.to_s)
         end
       end
   end
diff --git a/activemodel/test/cases/attribute_assignment_test.rb b/activemodel/test/cases/attribute_assignment_test.rb
index 30e8419685..4352714f12 100644
--- a/activemodel/test/cases/attribute_assignment_test.rb
+++ b/activemodel/test/cases/attribute_assignment_test.rb
@@ -49,8 +49,8 @@ class AttributeAssignmentTest < ActiveModel::TestCase
       @parameters
     end
 
-    def stringify_keys
-      dup
+    def each_pair(&block)
+      @parameters.each_pair(&block)
     end
 
     def dup
diff --git a/activerecord/lib/active_record/attribute_assignment.rb b/activerecord/lib/active_record/attribute_assignment.rb
index acb8ba7e5a..2882b1b583 100644
--- a/activerecord/lib/active_record/attribute_assignment.rb
+++ b/activerecord/lib/active_record/attribute_assignment.rb
@@ -6,16 +6,22 @@ module ActiveRecord
   module AttributeAssignment
     include ActiveModel::AttributeAssignment
 
+    def assign_attributes(attributes)
+      super(attributes.dup)
+    end
+
     private
       def _assign_attributes(attributes)
         multi_parameter_attributes  = {}
         nested_parameter_attributes = {}
 
         attributes.each do |k, v|
-          if k.include?("(")
-            multi_parameter_attributes[k] = attributes.delete(k)
+          key = k.to_s
+
+          if key.include?("(")
+            multi_parameter_attributes[key] = attributes.delete(k)
           elsif v.is_a?(Hash)
-            nested_parameter_attributes[k] = attributes.delete(k)
+            nested_parameter_attributes[key] = attributes.delete(k)
           end
         end
         super(attributes)
diff --git a/activerecord/test/support/stubs/strong_parameters.rb b/activerecord/test/support/stubs/strong_parameters.rb
index da8f9892f9..8e57c7e776 100644
--- a/activerecord/test/support/stubs/strong_parameters.rb
+++ b/activerecord/test/support/stubs/strong_parameters.rb
@@ -28,8 +28,8 @@ class ProtectedParams
   end
   alias to_unsafe_h to_h
 
-  def stringify_keys
-    dup
+  def each_pair(&block)
+    @parameters.each_pair(&block)
   end
 
   def dup