Index: lib/net/http.rb
===================================================================
--- lib/net/http.rb	(revision 23133)
+++ lib/net/http.rb	(working copy)
@@ -1517,7 +1517,7 @@
     #    http.set_form_data({"q" => "ruby", "lang" => "en"}, ';')
     #
     def set_form_data(params, sep = '&')
-      self.body = params.map {|k, v| encode_kvpair(k, v) }.flatten.join(sep)
+      self.body = params.map {|k, v| encode_kvpair(k.to_s, v) }.flatten.join(sep)
       self.content_type = 'application/x-www-form-urlencoded'
     end