diff --git a/Library/Homebrew/os/mac.rb b/Library/Homebrew/os/mac.rb
index 73dc51389b..6163d11f23 100644
--- a/Library/Homebrew/os/mac.rb
+++ b/Library/Homebrew/os/mac.rb
@@ -111,7 +111,7 @@ module OS
 
       # If there's no CLT SDK, return early
       return if MacOS::CLT.installed? && !MacOS::CLT.provides_sdk?
-      # If the CLT is installed and provides headers, return early
+      # If the CLT is installed and headers are provided by the system, return early
       return if MacOS::CLT.installed? && !MacOS::CLT.separate_header_package?
 
       sdk_path(v)
diff --git a/Library/Homebrew/os/mac/xcode.rb b/Library/Homebrew/os/mac/xcode.rb
index 2435a8f0dd..2a67e47d41 100644
--- a/Library/Homebrew/os/mac/xcode.rb
+++ b/Library/Homebrew/os/mac/xcode.rb
@@ -200,7 +200,7 @@ module OS
       end
 
       def separate_header_package?
-        version >= "10"
+        version >= "10" && MacOS.version >= "10.14"
       end
 
       def provides_sdk?