diff --git a/pri_internal.h b/pri_internal.h
index b0a7064..29e8c2f 100644
--- a/pri_internal.h
+++ b/pri_internal.h
@@ -25,6 +25,7 @@
 #ifndef _PRI_INTERNAL_H
 #define _PRI_INTERNAL_H
 
+#include <stddef.h>
 #include <sys/time.h>
 
 struct pri_sched {