diff --git a/activerecord/lib/active_record/insert_all.rb b/activerecord/lib/active_record/insert_all.rb
index fbdb67becb..fa88784e47 100644
--- a/activerecord/lib/active_record/insert_all.rb
+++ b/activerecord/lib/active_record/insert_all.rb
@@ -14,7 +14,7 @@ module ActiveRecord
       @returning = (connection.supports_insert_returning? ? primary_keys : false) if @returning.nil?
       @returning = false if @returning == []
 
-      @unique_by = find_unique_index_for(unique_by || model.primary_key)
+      @unique_by = find_unique_index_for(unique_by)
       @on_duplicate = :skip if @on_duplicate == :update && updatable_columns.empty?
 
       ensure_valid_options_for_connection!
@@ -57,14 +57,15 @@ module ActiveRecord
 
     private
       def find_unique_index_for(unique_by)
-        match = Array(unique_by).map(&:to_s)
+        name_or_columns = unique_by || model.primary_key
+        match = Array(name_or_columns).map(&:to_s)
 
         if index = unique_indexes.find { |i| match.include?(i.name) || i.columns == match }
           index
         elsif match == primary_keys
-          nil
+          unique_by.nil? ? nil : ActiveRecord::ConnectionAdapters::IndexDefinition.new(model.table_name, "#{model.table_name}_primary_key", true, match)
         else
-          raise ArgumentError, "No unique index found for #{unique_by}"
+          raise ArgumentError, "No unique index found for #{name_or_columns}"
         end
       end
 
diff --git a/activerecord/test/cases/insert_all_test.rb b/activerecord/test/cases/insert_all_test.rb
index 294d40de1e..08ab0ae06b 100644
--- a/activerecord/test/cases/insert_all_test.rb
+++ b/activerecord/test/cases/insert_all_test.rb
@@ -155,10 +155,11 @@ class InsertAllTest < ActiveRecord::TestCase
   def test_insert_all_and_upsert_all_with_index_finding_options
     skip unless supports_insert_conflict_target?
 
-    assert_difference "Book.count", +3 do
+    assert_difference "Book.count", +4 do
       Book.insert_all [{ name: "Rework", author_id: 1 }], unique_by: :isbn
       Book.insert_all [{ name: "Remote", author_id: 1 }], unique_by: %i( author_id name )
       Book.insert_all [{ name: "Renote", author_id: 1 }], unique_by: :index_books_on_isbn
+      Book.insert_all [{ name: "Recoat", author_id: 1 }], unique_by: :id
     end
 
     assert_raise ActiveRecord::RecordNotUnique do
@@ -227,7 +228,7 @@ class InsertAllTest < ActiveRecord::TestCase
   end
 
   def test_upsert_all_updates_existing_record_by_primary_key
-    skip unless supports_insert_on_duplicate_update?
+    skip unless supports_insert_conflict_target?
 
     Book.upsert_all [{ id: 1, name: "New edition" }], unique_by: :id