diff --git a/lib/erb/compiler.rb b/lib/erb/compiler.rb
index 8a4d85c..6d70288 100644
--- a/lib/erb/compiler.rb
+++ b/lib/erb/compiler.rb
@@ -225,7 +225,7 @@ def explicit_trim_line(line)
       end
     end
 
-    ERB_STAG = %w(<%= <%# <%)
+    ERB_STAG = %w(<%= <%# <%).freeze
     def is_erb_stag?(s)
       ERB_STAG.member?(s)
     end