diff --git a/src/variable.c b/src/variable.c
index a07412684..598850034 100644
--- a/src/variable.c
+++ b/src/variable.c
@@ -486,7 +486,7 @@ mrb_value
 mrb_obj_iv_inspect(mrb_state *mrb, struct RObject *obj)
 {
   iv_tbl *t = obj->iv;
-  int len = iv_size(mrb, t);
+  size_t len = iv_size(mrb, t);
 
   if (len > 0) {
     const char *cn = mrb_obj_classname(mrb, mrb_obj_value(obj));