diff --git a/line.c b/line.c
index cfa636f..0826029 100644
--- a/line.c
+++ b/line.c
@@ -16,6 +16,7 @@
 #include "line.h"
 
 #include <assert.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>