当我在发件人一侧关闭时，为什么仍会慌张地“在关闭的通道上发送”？

我有一个stopChan通知发件人关闭频道的通知sync.Once，并确保只有一个发件人可以关闭频道，但是我仍然收到“在关闭的频道上发送”的恐慌，为什么？

func muitiSenderClose() {
    const SenderNum = 3

    wg := sync.WaitGroup{}
    wg.Add(SenderNum)

    intChan := make(chan int)

    stopChan := make(chan struct{})

    once := sync.Once{}
    for i := 0; i < SenderNum; i++ {
        go func(i int) {
            defer wg.Done()
            needStop := false
            for {
                select {
                case <-stopChan:
                    needStop = true
                case intChan <- 1:
                case intChan <- 2:
                case intChan <- 3:
                }

                if needStop {
                    break
                }
            }
            once.Do(func() {
                fmt.Printf("%d want to close chan\n",i)
                close(intChan)
            })
            fmt.Println("End. [sender] %id", i)
        }(i)
    }
    sum := 0
    for e := range intChan {
        fmt.Printf("Receive %d\n", e)
        sum += e
        if sum > 10 {
            close(stopChan)
            fmt.Printf("Got %d\n", sum)
            break
        }
    }
    fmt.Println("End. [receiver]")

    wg.Wait()
}