diff --git a/lib/uh/wm/env.rb b/lib/uh/wm/env.rb
index 1b3c00a..51ffc34 100644
--- a/lib/uh/wm/env.rb
+++ b/lib/uh/wm/env.rb
@@ -11,7 +11,7 @@ module Uh
       LOGGER_LEVEL          = Logger::WARN
       LOGGER_LEVEL_VERBOSE  = Logger::INFO
       LOGGER_LEVEL_DEBUG    = Logger::DEBUG
-      LOGGER_LEVEL_STRINGS  = %w[DEBUG INFO WARN ERROR FATAL UNKNOWN]
+      LOGGER_LEVEL_STRINGS  = %w[DEBUG INFO WARN ERROR FATAL UNKNOWN].freeze
 
       extend Forwardable
       def_delegator :logger, :info, :log