diff --git a/src/vm.c b/src/vm.c
index cca0fd03b..3023710d4 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -134,8 +134,8 @@ static void
 stack_extend_alloc(mrb_state *mrb, int room, int keep)
 {
   mrb_value *oldbase = mrb->c->stbase;
-  int size = mrb->c->stend - mrb->c->stbase;
-  int off = mrb->c->stack - mrb->c->stbase;
+  size_t size = mrb->c->stend - mrb->c->stbase;
+  size_t off = mrb->c->stack - mrb->c->stbase;
 
 #ifdef MRB_STACK_EXTEND_DOUBLING
   if (room <= size)