diff --git a/runtime/syntax/c.vim b/runtime/syntax/c.vim
index 3648860..7925e88 100644
--- a/runtime/syntax/c.vim
+++ b/runtime/syntax/c.vim
@@ -1,7 +1,7 @@
 " Vim syntax file
 " Language:	C
 " Maintainer:	Bram Moolenaar <Bram@vim.org>
-" Last Change:	2020 Aug 11
+" Last Change:	2020 Aug 28
 
 " Quit when a (custom) syntax file was already loaded
 if exists("b:current_syntax")
@@ -346,6 +346,8 @@
   syn keyword cConstant SIGSTOP SIGTERM SIGTRAP SIGTSTP SIGTTIN SIGTTOU SIGUSR1 SIGUSR2
   syn keyword cConstant _IOFBF _IOLBF _IONBF BUFSIZ EOF WEOF FOPEN_MAX FILENAME_MAX L_tmpnam
   syn keyword cConstant SEEK_CUR SEEK_END SEEK_SET TMP_MAX stderr stdin stdout EXIT_FAILURE EXIT_SUCCESS RAND_MAX
+  " used in assert.h
+  syn keyword cConstant NDEBUG
   " POSIX 2001
   syn keyword cConstant SIGBUS SIGPOLL SIGPROF SIGSYS SIGURG SIGVTALRM SIGXCPU SIGXFSZ
   " non-POSIX signals