diff --git a/Library/Homebrew/Gemfile b/Library/Homebrew/Gemfile
index f7f910caa8..a434b3a5c9 100644
--- a/Library/Homebrew/Gemfile
+++ b/Library/Homebrew/Gemfile
@@ -2,6 +2,9 @@
 
 source "https://rubygems.org"
 
+# disallowed gems (should not be used)
+# * nokogiri - use rexml instead for XML parsing
+
 # installed gems (should all be require: false)
 gem "bootsnap", require: false
 gem "byebug", require: false