diff --git a/src/userprog/syscall.c b/src/userprog/syscall.c
index ddbbd39..fc8c28e 100644
--- a/src/userprog/syscall.c
+++ b/src/userprog/syscall.c
@@ -80,11 +80,15 @@ exit (int status UNUSED)
 }
 
 /* Validates if a block of memory starting at PTR and of size SIZE bytes is
-   fully contained within user virtual memory. */
+   fully contained within user virtual memory. Kills the thread (by calling
+   thread_exit) if the memory is invalid. Otherwise, returns the PTR given.
+   If the size is 0, the function does no checks and returns PTR.*/
 static void *
 validate_user_pointer (void *ptr, size_t size)
 {
-  if (ptr == NULL || !is_user_vaddr (ptr) || !is_user_vaddr (ptr + size - 1))
+  if (size > 0 && (ptr == NULL ||
+                   !is_user_vaddr (ptr) ||
+                   !is_user_vaddr (ptr + size - 1)))
     thread_exit ();
 
   return ptr;