diff --git a/lib/liquid/parser.rb b/lib/liquid/parser.rb
index 152e04c..2bc4f95 100644
--- a/lib/liquid/parser.rb
+++ b/lib/liquid/parser.rb
@@ -44,7 +44,7 @@ module Liquid
       tok[0] == type
     end
 
-    CONSUME_TOKENS = [:string, :number]
+    CONSUME_TOKENS = [:string, :number].freeze
     private_constant :CONSUME_TOKENS
 
     def expression