Please, consider applying the following patch to allow empty values in a key/value query string. There are many situations in which they can occur.

Regards,
F. Moya

diff --git a/boost/network/uri/accessors.hpp b/boost/network/uri/accessors.hpp
index c050458..5248b34 100644
--- a/boost/network/uri/accessors.hpp
+++ b/boost/network/uri/accessors.hpp
@@ -27,7 +27,7 @@ struct key_value_sequence : spirit::qi::grammar {
     pair = key >> -('=' >> value);
     key =
         spirit::qi::char_("a-zA-Z_") >> *spirit::qi::char_("-+.~a-zA-Z_0-9/%");
-    value = +spirit::qi::char_("-+.~a-zA-Z_0-9/%");
+    value = *spirit::qi::char_("-+.~a-zA-Z_0-9/%");
   }
 
   spirit::qi::rule query;