diff --git a/bacnet-stack/bacdcode.h b/bacnet-stack/bacdcode.h
index 63146218..aa68817e 100644
--- a/bacnet-stack/bacdcode.h
+++ b/bacnet-stack/bacdcode.h
@@ -36,6 +36,7 @@
 
 #include <stdint.h>
 #include <stdbool.h>
+#include <stddef.h>
 
 // from clause 20.2.1 General Rules for Encoding BACnet Tags
 // returns the number of apdu bytes consumed