diff --git a/lib/uri/common.rb b/lib/uri/common.rb
index 764f89d..4fed901 100644
--- a/lib/uri/common.rb
+++ b/lib/uri/common.rb
@@ -376,7 +376,7 @@ def self.encode_www_form_component(str, enc=nil)
     if str.encoding != Encoding::ASCII_8BIT
       if enc && enc != Encoding::ASCII_8BIT
         str.encode!(Encoding::UTF_8, invalid: :replace, undef: :replace)
-        str.encode!(enc, fallback: ->(x){"&#{x.ord};"})
+        str.encode!(enc, fallback: ->(x){"&##{x.ord};"})
       end
       str.force_encoding(Encoding::ASCII_8BIT)
     end