diff --git a/vm.c b/vm.c
index d9aaa7e..764512c 100644
--- a/vm.c
+++ b/vm.c
@@ -266,7 +266,7 @@ deallocuvm(pde_t *pgdir, uint oldsz, uint newsz)
   for(; a  < oldsz; a += PGSIZE){
     pte = walkpgdir(pgdir, (char*)a, 0);
     if(!pte)
-      a += (NPTENTRIES - 1) * PGSIZE;
+      a = PGADDR(PDX(a) + 1, 0, 0) - PGSIZE;
     else if((*pte & PTE_P) != 0){
       pa = PTE_ADDR(*pte);
       if(pa == 0)