diff --git a/activerecord/lib/active_record/attribute_methods/primary_key.rb b/activerecord/lib/active_record/attribute_methods/primary_key.rb
index 16362acfe955fc210ed2b2d9780d9d0f9be50536..6eff7167034019fc775a2a77377b599721aea7db 100644
--- a/activerecord/lib/active_record/attribute_methods/primary_key.rb
+++ b/activerecord/lib/active_record/attribute_methods/primary_key.rb
@@ -33,7 +33,8 @@ def dangerous_attribute_method?(method_name)
         # Defines the primary key field -- can be overridden in subclasses. Overwriting will negate any effect of the
         # primary_key_prefix_type setting, though.
         def primary_key
-          @primary_key ||= reset_primary_key
+          @primary_key = reset_primary_key unless defined? @primary_key
+          @primary_key
         end
 
         # Returns a quoted version of the primary key name, used to construct SQL statements.
diff --git a/activerecord/test/cases/attribute_methods_test.rb b/activerecord/test/cases/attribute_methods_test.rb
index 12e5715710fc82bf8a32bebe604f67770715b1c7..7c9383b194239ef6f9181b9c9c5dca370fc3b27a 100644
--- a/activerecord/test/cases/attribute_methods_test.rb
+++ b/activerecord/test/cases/attribute_methods_test.rb
@@ -53,6 +53,11 @@ def test_attribute_present_with_booleans
     assert Boolean.find(b4.id).attribute_present?(:value)
   end
 
+  def test_caching_nil_primary_key
+    Minimalistic.expects(:reset_primary_key).returns(nil).once
+    Minimalistic.create!
+  end
+
   def test_attribute_keys_on_new_instance
     t = Topic.new
     assert_equal nil, t.title, "The topics table has a title column, so it should be nil"