diff --git a/userprog/syscall_open.c b/userprog/syscall_open.c
index 8855a8b5e1a529fadfd23266a4afe2565fe1f80e..fadce9511dec77c50fa7f8e10baacd6843d7b873 100644
--- a/userprog/syscall_open.c
+++ b/userprog/syscall_open.c
@@ -7,6 +7,7 @@
 
 
 #include "system_calls.h"
+#include <stddef.h> // Dependency for NULL
 #include "threads/interrupt.h" // Dependency for intr_frame struct
 #include "filesys/file.h" // Dependency for and file struct
 #include "filesys/filesys.h" // Dependency for filesys_open