diff --git a/src/vm.c b/src/vm.c
index 7dd9d5d6d..456035c0c 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -96,7 +96,7 @@ envadjust(mrb_state *mrb, mrb_value *oldbase, mrb_value *newbase)
   while (ci <= mrb->ci) {
     struct REnv *e = ci->env;
     if (e && e->cioff >= 0) {
-      int off = e->stack - oldbase;
+      ptrdiff_t off = e->stack - oldbase;
 
       e->stack = newbase + off;
     }