diff --git a/src/string.c b/src/string.c
index 5cca6d460..fd09e05dc 100644
--- a/src/string.c
+++ b/src/string.c
@@ -708,6 +708,9 @@ mrb_str_resize(mrb_state *mrb, mrb_value str, mrb_int len)
   mrb_int slen;
   struct RString *s = mrb_str_ptr(str);
 
+  if (len < 0) {
+    mrb_raisef(mrb, E_ARGUMENT_ERROR, "negative (or overflowed) string size");
+  }
   mrb_str_modify(mrb, s);
   slen = RSTR_LEN(s);
   if (len != slen) {