diff --git a/include/mruby/irep.h b/include/mruby/irep.h
index 2a0e261d1..8382c3b18 100644
--- a/include/mruby/irep.h
+++ b/include/mruby/irep.h
@@ -122,7 +122,7 @@ struct mrb_insn_data {
   uint8_t insn;
   uint16_t a;
   uint16_t b;
-  uint8_t c;
+  uint16_t c;
   const mrb_code *addr;
 };