diff --git a/Library/Homebrew/cmd/update.sh b/Library/Homebrew/cmd/update.sh
index d557ad2f3..111644907 100644
--- a/Library/Homebrew/cmd/update.sh
+++ b/Library/Homebrew/cmd/update.sh
@@ -390,6 +390,22 @@ EOS
     fi
   fi
 
+  if [[ -f "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-core/.git/shallow" &&
+      -f "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-cask/.git/shallow" ]]
+  then
+    odie <<EOS
+homebrew-core and homebrew-cask are shallow clones. To \`brew update\` first run:
+  git -C "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-core" fetch --unshallow
+  git -C "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-cask" fetch --unshallow
+This restriction has been made on GitHub's request because updating shallow
+clones is an extremely expensive operation due to the tree layout and traffic of
+Homebrew/homebrew-core and Homebrew/homebrew-core. We don't do this for you
+automatically to avoid repeatedly performing an expensive unshallow operation in
+CI systems (which should instead be fixed to not use shallow clones). Sorry for
+the inconvenience!
+EOS
+  fi
+
   if [[ -f "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-core/.git/shallow" ]]
   then
     odie <<EOS