diff --git a/libmctp.h b/libmctp.h
index 8a66ef8..96e1076 100644
--- a/libmctp.h
+++ b/libmctp.h
@@ -5,6 +5,7 @@
 
 #include <stdbool.h>
 #include <stdint.h>
+#include <stddef.h>
 
 typedef uint8_t mctp_eid_t;