PlatformIO's serial monitor reads NULL instead characters

I’m using an Esp-01 and an usb to ttl serial converter for serial communication.
If I use the PlatformIO’s buildt in serial monitor, it only shows NULL’s instead of characters, but if I try it with the same BAUD-rate at Arduino IDE, it works correctly.
What’s the problem with the PlatfomIO’s serial monitor, what do I need to change in the setups?

Thanks: T

EDIT: It seem’s to appear only in this code, and maybe caused by the “BUILTIN_LED” constant.

#include <Arduino.h>
void setup() {
 void loop() {
  digitalWrite(BUILTIN_LED, HIGH);
  delay(1000);              // wait for a second
  digitalWrite(BUILTIN_LED, LOW);
  delay(1000);              // wait for a second

If you need higher baud rates you need to change the baud rate in Serial.begin(speed)

@valeros could you re-test it?

Yes, of course:
The result is, that I was wrong, and set a wrog baud-rate. After changing to correct baud, it works perfectly.