diff --git a/book.toml b/book.toml
index 5b2808637..097706087 100644
--- a/book.toml
+++ b/book.toml
@@ -41,7 +41,7 @@ exclude = [
     "tomlee\\.co",
     "marketplace\\.visualstudio\\.com",
     "objects\\.githubusercontent\\.com",
-    # The bug listing URL works only if an user is logged in.
+    # The bug listing URL works only if an user is logged in, otherwise one gets 404.
     "github\\.com/issues\\?",
     # Handle: connection closed before message completed
     "microsoft\\.com/en-us/research/publication/",