diff --git a/lib/mongoid/history/trackable.rb b/lib/mongoid/history/trackable.rb
index ce2b296b..d9a44e13 100644
--- a/lib/mongoid/history/trackable.rb
+++ b/lib/mongoid/history/trackable.rb
@@ -12,7 +12,7 @@ def track_history(options = {})
 
           field options[:version_field].to_sym, type: Integer
 
-          belongs_to_modifier_options = { class_name: Mongoid::History.modifier_class_name }
+          belongs_to_modifier_options = { class_name: Mongoid::History.modifier_class_name, optional: true }
           belongs_to_modifier_options[:inverse_of] = options[:modifier_field_inverse_of] if options.key?(:modifier_field_inverse_of)
           belongs_to options[:modifier_field].to_sym, belongs_to_modifier_options