diff --git a/mrbgems/mruby-binding/src/binding.c b/mrbgems/mruby-binding/src/binding.c
index eb44f0e90..e4c362793 100644
--- a/mrbgems/mruby-binding/src/binding.c
+++ b/mrbgems/mruby-binding/src/binding.c
@@ -45,7 +45,7 @@ binding_eval_error_check(mrb_state *mrb, struct mrb_parser_state *p, const char
 struct expand_lvspace {
   mrb_irep *irep;
   struct REnv *env;
-  size_t numvar;
+  int numvar;
   mrb_sym syms[LV_BUFFERS];
 };