diff --git a/lib/recurrence/event/yearly.rb b/lib/recurrence/event/yearly.rb
index eab5ba0..c7ca6f4 100644
--- a/lib/recurrence/event/yearly.rb
+++ b/lib/recurrence/event/yearly.rb
@@ -19,7 +19,7 @@ class Yearly < Base # :nodoc: all
       }.freeze
 
       private def validate
-        valid_month_day?(@options[:on].last)
+        valid_month_day?(@options[:on]&.last)
 
         if @options[:on].first.is_a?(Numeric)
           valid_month?(@options[:on].first)