Ktor: cause of cancelled, upstream prematurely closed?

I’m seeing this occasionally in my nginx log:

13:05:20 [error] 4144#0: *7074 upstream prematurely closed connection while sending to client, client: 192…, server: …, request: “GET /static/kotlin.js HTTP/1.1”, …

In the ktor log I see “cancelled

13:05:20.541 [nettyCallPool-4-1] INFO ktor.application - 200 OK: GET - /static/kotlin.js, cancelled

Any idea why that happens? This is on my dev machine, an old Macbook Pro. I haven’t used this app yet enough on the “live” Linux servers to see if it shows up there too.

routing {
    static("/static") {

ktor version 1.2.1