diff --git a/arena.h b/arena.h
index 18add9f..ec9ae52 100644
--- a/arena.h
+++ b/arena.h
@@ -116,6 +116,7 @@ int main(void)
 #ifndef ARENA_H_
 #define ARENA_H_
 
+#include <stdalign.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <string.h>