diff --git a/clang/include/clang/Config/config.h.cmake b/clang/include/clang/Config/config.h.cmake
index 40a6cb3a2e3e..7629ef9129b1 100644
--- a/clang/include/clang/Config/config.h.cmake
+++ b/clang/include/clang/Config/config.h.cmake
@@ -1,4 +1,8 @@
-#ifndef CONFIG_H
+/* This generated file is for internal use. Do not include it from headers. */
+
+#ifdef CONFIG_H
+#error config.h can only be included once
+#else
 #define CONFIG_H
 
 /* Bug report URL. */
diff --git a/clang/include/clang/Config/config.h.in b/clang/include/clang/Config/config.h.in
index 450ea9b613e8..1530fefaa53d 100644
--- a/clang/include/clang/Config/config.h.in
+++ b/clang/include/clang/Config/config.h.in
@@ -1,4 +1,8 @@
-#ifndef CONFIG_H
+/* This generated file is for internal use. Do not include it from headers. */
+
+#ifdef CONFIG_H
+#error config.h can only be included once
+#else
 #define CONFIG_H
 
 /* Bug report URL. */