diff --git a/src/userprog/process.c b/src/userprog/process.c
index 20e2677..4f72dc0 100644
--- a/src/userprog/process.c
+++ b/src/userprog/process.c
@@ -20,6 +20,7 @@
 #include "threads/thread.h"
 #include "threads/vaddr.h"
 #include "threads/synch.h"
+#include "devices/timer.h"
 
 /* Keeps track of the position of pointers to user program arguments
    within a linked list. */