diff --git a/lib/chronic/mini_date.rb b/lib/chronic/mini_date.rb
index 921fc2a0..b55d2498 100644
--- a/lib/chronic/mini_date.rb
+++ b/lib/chronic/mini_date.rb
@@ -7,7 +7,7 @@ def self.from_time(time)
     end
 
     def initialize(month, day)
-      unless (1..12).include?(month)
+      unless (1..12).cover?(month)
         raise ArgumentError, '1..12 are valid months'
       end