diff --git a/tool/lib/envutil.rb b/tool/lib/envutil.rb
index c620b8dbdb4..1b548b0cbf9 100644
--- a/tool/lib/envutil.rb
+++ b/tool/lib/envutil.rb
@@ -518,7 +518,7 @@ end
 
 if defined?(RbConfig)
   module RbConfig
-    @ruby = EnvUtil.rubybin
+    @ruby = EnvUtil.rubybin.freeze
     class << self
       undef ruby if method_defined?(:ruby)
       attr_reader :ruby