diff --git a/lib/castle/configuration.rb b/lib/castle/configuration.rb
index 3672e535..012104ec 100644
--- a/lib/castle/configuration.rb
+++ b/lib/castle/configuration.rb
@@ -41,8 +41,8 @@ def initialize
       self.host = HOST
       self.port = PORT
       self.url_prefix = URL_PREFIX
-      self.whitelisted = []
-      self.blacklisted = []
+      self.whitelisted = [].freeze
+      self.blacklisted = [].freeze
       self.api_secret = ''
     end