After mbed update --clean, serial port doesn't work anymore

Mbed OS
1

I changed mbed-os-example-blinky, so it prints on the serial port.
But after a ‘mbed update --clean’, it no longer sends out to the serial port.
Why is that?

#include "mbed.h"

DigitalOut led1(LED1);
Serial pc(USBTX, USBRX);

// main() runs in its own thread in the OS
int main() {
    int i = 0;

    while (true) {

        pc.printf("%d\r\n", i);
        i++;

        led1 = !led1;
        wait(0.5);
    }
}