diff --git a/libc/include/sys/stat.h b/libc/include/sys/stat.h
index 46eec96..f26a492 100644
--- a/libc/include/sys/stat.h
+++ b/libc/include/sys/stat.h
@@ -157,7 +157,7 @@
 extern int mknod(const char*, mode_t, dev_t);
 extern mode_t umask(mode_t);
 
-extern mode_t __umask_chk(mode_t) __INTRODUCED_IN(21);
+extern mode_t __umask_chk(mode_t) __INTRODUCED_IN(18);
 extern mode_t __umask_real(mode_t) __RENAME(umask);
 __errordecl(__umask_invalid_mode, "umask called with invalid mode");