diff --git a/Library/Homebrew/brew.sh b/Library/Homebrew/brew.sh
index df0eda2a3bdb2af9c9c803412682ccf6e5bc41cd..d448ca333700859036e69bdf581b760fd3d05c46 100644
--- a/Library/Homebrew/brew.sh
+++ b/Library/Homebrew/brew.sh
@@ -341,7 +341,9 @@ then
 
   # Set a variable when the macOS system Ruby is new enough to avoid spawning
   # a Ruby process unnecessarily.
-  if [[ "$HOMEBREW_MACOS_VERSION_NUMERIC" -lt "101500" ]]
+  # On Catalina the system Ruby is technically new enough but don't allow it:
+  # https://github.com/Homebrew/brew/issues/9410
+  if [[ "$HOMEBREW_MACOS_VERSION_NUMERIC" -lt "101600" ]]
   then
     unset HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH
   else