diff --git a/src/userprog/process.c b/src/userprog/process.c
index 0e9fc75..01f30d5 100644
--- a/src/userprog/process.c
+++ b/src/userprog/process.c
@@ -147,7 +147,7 @@ start_process (void *file_name_)
   
   /* Push a null pointer sentinel inside argv. */
   if_.esp -= sizeof (char *);
-  *(char *) if_.esp = NULL;
+  *(char *) if_.esp = 0;
   
   /* Push pointer to the process file name to the stack. */
   char **argv;
@@ -173,7 +173,7 @@ start_process (void *file_name_)
 
   /* Push fake return address (null pointer). */
   if_.esp -= sizeof (char *);
-  *(char *) if_.esp = NULL;
+  *(char *) if_.esp = 0;
 
   /* If load failed, quit. */
   palloc_free_page (file_name_);