Add assertion as originally intended; ref
@@ -1493,6 +1493,7 @@ mrb_io_readchar(mrb_state *mrb, mrb_value self)
#endif
mrb_get_args(mrb, "S", &buf);
+ mrb_assert(RSTRING_LEN(buf) > 0);
mrb_assert(RSTRING_PTR(buf) != NULL);
mrb_str_modify(mrb, RSTRING(buf));
#ifdef MRB_UTF8_STRING
