diff --git a/dlls/nsiproxy.sys/udp.c b/dlls/nsiproxy.sys/udp.c
index 49acedcf214..b6109ef9592 100644
--- a/dlls/nsiproxy.sys/udp.c
+++ b/dlls/nsiproxy.sys/udp.c
@@ -20,6 +20,7 @@
  */
 #include "config.h"
 #include <stdarg.h>
+#include <stddef.h>
 
 #ifdef HAVE_SYS_SYSCTL_H
 #include <sys/sysctl.h>