diff --git a/bacnet-stack/demo/writeprop/main.c b/bacnet-stack/demo/writeprop/main.c
index 78b29acf..6214ef15 100644
--- a/bacnet-stack/demo/writeprop/main.c
+++ b/bacnet-stack/demo/writeprop/main.c
@@ -31,6 +31,7 @@
 #include <time.h>               /* for time */
 #include <string.h>
 #include <errno.h>
+#include <ctype.h> /* toupper */
 #include "bactext.h"
 #include "iam.h"
 #include "arf.h"