diff --git a/src/string.c b/src/string.c
index 7cc9c4ff3..e444d8ecf 100644
--- a/src/string.c
+++ b/src/string.c
@@ -2632,7 +2632,7 @@ mrb_str_cat(mrb_state *mrb, mrb_value str, const char *ptr, size_t len)
 {
   struct RString *s = mrb_str_ptr(str);
   mrb_int capa;
-  size_t total;
+  mrb_int total;
   ptrdiff_t off = -1;
 
   if (len == 0) return str;