diff --git a/activerecord/lib/active_record/associations/collection_proxy.rb b/activerecord/lib/active_record/associations/collection_proxy.rb
index e044099b72ffd30eb12b021f1b638d945f4f9462..1b746b46b72a6557a8843a6f1612f6b9f95f0800 100644
--- a/activerecord/lib/active_record/associations/collection_proxy.rb
+++ b/activerecord/lib/active_record/associations/collection_proxy.rb
@@ -28,6 +28,8 @@ module Associations
     # is computed directly through SQL and does not trigger by itself the
     # instantiation of the actual post records.
     class CollectionProxy < Relation
+      delegate *ActiveRecord::Calculations.public_instance_methods, to: :scope
+
       def initialize(association) #:nodoc:
         @association = association
         super association.klass, association.klass.arel_table
@@ -965,10 +967,6 @@ def reload
         proxy_association.reload
         self
       end
-
-      def pluck(column_names)
-        scope.pluck(column_names)
-      end
     end
   end
 end
diff --git a/activerecord/lib/active_record/null_relation.rb b/activerecord/lib/active_record/null_relation.rb
index f8f7a6c9769618179a2b5b31f5828abccc1139ae..711fc8b883ca189d5f948c9b7dd6a866ba61054f 100644
--- a/activerecord/lib/active_record/null_relation.rb
+++ b/activerecord/lib/active_record/null_relation.rb
@@ -50,6 +50,10 @@ def count(*)
       0
     end
 
+    def sum(*)
+      0
+    end
+
     def calculate(_operation, _column_name, _options = {})
       nil
     end
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index baf07f352e35294b3925c7b1b2eb090590a615c1..aae93bb9d0bb7a947290f9fd9374f96dfcd08e25 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -1656,6 +1656,7 @@ def test_collection_association_with_private_kernel_method
       assert_equal [], post.comments
       assert_equal [], post.comments.where(body: 'omg')
       assert_equal [], post.comments.pluck(:body)
+      assert_equal 0,  post.comments.sum(:id)
     end
   end
 end