Index: UI/MailerUI/UIxMailView.m
--- UI/MailerUI/UIxMailView.m.orig
+++ UI/MailerUI/UIxMailView.m
@@ -339,7 +339,7 @@ static NSString *mailETag = nil;
   
   if (![self message]) // TODO: redirect to proper error
     {
-      data = [NSDictionary dictionaryWithObject: [self labelForKey: @"Did not find specified message"]
+      data = [NSMutableDictionary dictionaryWithObject: [self labelForKey: @"Did not find specified message"]
                                          forKey: @"message"];
       return [self responseWithStatus: 404 /* Not Found */
                 andJSONRepresentation: data];
