diff --git a/src/core/ipv4/inet_chksum.c b/src/core/ipv4/inet_chksum.c
index dea8d5f2..185881ef 100644
--- a/src/core/ipv4/inet_chksum.c
+++ b/src/core/ipv4/inet_chksum.c
@@ -41,6 +41,8 @@
 #include "lwip/inet_chksum.h"
 #include "lwip/inet.h"
 
+#include <stddef.h>
+
 /* These are some reference implementations of the checksum algorithm, with the
  * aim of being simple, correct and fully portable. Checksumming is the
  * first thing you would want to optimize for your platform. If you create