diff --git a/src/vm.c b/src/vm.c
index a1f272a52..96d9266d8 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -134,19 +134,29 @@ static inline void
 envadjust(mrb_state *mrb, mrb_value *oldbase, mrb_value *newbase)
 {
   mrb_callinfo *ci = mrb->c->cibase;
-  ptrdiff_t delta = newbase - oldbase;
+  /*
+   * Byte-level calculation to avoid truncation when allocator alignment is
+   * smaller than sizeof(mrb_value).
+   * eg: MRB_NO_BOXING + MRB_INT64 with MRB_32BIT => sizeof(mrb_value)=16
+   *     And when memory allocator's alignment is 8 bytes
+   * Pointer subtraction on mrb_value* would truncate (8/16 -> 0).
+   * So, we use char* for pointer calculation to get the correct offset in bytes,
+   * then apply that offset to mrb_value* pointers.
+   */
+  ptrdiff_t off = (char *)newbase - (char *)oldbase;
 
-  if (delta == 0) return;
+  if (off == 0) return;
   while (ci <= mrb->c->ci) {
     struct REnv *e = mrb_vm_ci_env(ci);
 
+    mrb_value *new_stack = (mrb_value *)((char *)ci->stack + off);
+
     if (e) {
       mrb_assert(e->cxt == mrb->c && MRB_ENV_ONSTACK_P(e));
       mrb_assert(e->stack == ci->stack);
-
-      e->stack += delta;
+      e->stack = new_stack;
     }
-    ci->stack += delta;
+    ci->stack = new_stack;
     ci++;
   }
 }