diff --git a/www/%participant_id/index.html b/www/%participant_id/index.html
index de63c2b5d4..649f2434f5 100644
--- a/www/%participant_id/index.html
+++ b/www/%participant_id/index.html
@@ -281,7 +281,7 @@
 
                 function success(d)
                 {
-                    window.location.href = "/" + encodeURI(d.participant_id) + "/";
+                    window.location.href = "/" + encodeURIComponent(d.participant_id) + "/";
                 }
                 function error(e)
                 {
@@ -297,8 +297,9 @@
                     }
                     else
                     {
-                        alert( "Sorry, something went wrong. :-( Try again "
-                             + "later?");
+                        alert( "Sorry, something went wrong. Either you used "
+                             + "disallowed characters or something broke on "
+                             + "our end.");
                     }
                 }
                 jQuery.ajax(
diff --git a/www/%participant_id/participant_id.json b/www/%participant_id/participant_id.json
index 23e9cfc3a5..b67aff9062 100644
--- a/www/%participant_id/participant_id.json
+++ b/www/%participant_id/participant_id.json
@@ -2,6 +2,12 @@ from aspen import Response
 from gittip import db
 from psycopg2 import IntegrityError
 
+ALLOWED_ASCII = set("0123456789"
+                    "abcdefghijklmnopqrstuvwxyz"
+                    "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                    ".,-_;:@ ")
+
+
 # ========================================================================== ^L
 
 if user.ANON:
@@ -18,8 +24,11 @@ new_participant_id = request.body['participant_id']
 for i, c in enumerate(new_participant_id):
     if i == 32:
         raise Response(413)  # Request Entity Too Large (more or less)
-    if ord(c) < 32:
+    elif ord(c) < 128 and c not in ALLOWED_ASCII:
         raise Response(400)  # Yeah, no.
+    elif c not in ALLOWED_ASCII:
+        raise Response(400)  # XXX Burned by an Aspen bug. :`-(
+                             # https://github.com/whit537/aspen/issues/102
 
 
 # Persist