diff --git a/src/brei-shared.h b/src/brei-shared.h
index 80c3899..108ff39 100644
--- a/src/brei-shared.h
+++ b/src/brei-shared.h
@@ -25,6 +25,8 @@
 
 #include "config.h"
 
+#include <stddef.h>
+
 struct brei_message {
 	const char *data;
 	size_t len;