diff --git a/lib/payjp/util.rb b/lib/payjp/util.rb
index 69b0569..77bdfa0 100644
--- a/lib/payjp/util.rb
+++ b/lib/payjp/util.rb
@@ -71,7 +71,7 @@ def self.url_encode(key)
       # (from URI::RFC2396_Parser#escape)
       key.to_s.gsub(Regexp.new("[^#{URI::PATTERN::UNRESERVED}]")) do
         us = $&
-        tmp = ''
+        tmp = String.new
         us.each_byte do |uc|
           tmp << sprintf('%%%02X', uc)
         end