Why does the kernel quit if I want to use minus a sum that I converted from a list?

Question

Using Mathematica 11 when I define a list

yy = {x*43, 32*y};

and then convert it to a sum

yy[[0]] = Plus;
yy

I get, as expected, x*43 + 32*y.

However, as soon as I try to multiply my sum with a minus sign, i.e. when I execute

-yy

my kernel quits without any error message.

Any ideas why this happens and how to avoid it?

---

The problem can be avoided by converting the list using a Do loop:

sum = 0;
Do[
  sum = sum + yy[[i]],
  {i, Length[yy]}
  ];
sum
-sum
(* 43 x + 32 y *)
(* -43 x - 32 y *)