"?" symbols instead of NewPing readings (HC_SR04)

Hi there,

I'm using the sample code from the NewPing library to control 3 HC_SR04 sensors.

Even though the code works as intended the symbols displayed are not the distance readings.
How can this be corrected? Is it a glitch in the library?

Thanks in advance,
Charles

#include <NewPing.h>

#define SONAR_NUM     3 // Number or sensors.
#define MAX_DISTANCE 400 // Max distance in cm.
#define PING_INTERVAL 33 // Milliseconds between pings.
#define LEDL 6
#define LEDM 6
#define LEDR 6
int LEDS[] = {5, 4, 6};

unsigned long pingTimer[SONAR_NUM]; // When each pings.
unsigned int cm[SONAR_NUM]; // Store ping distances.
uint8_t currentSensor = 0; // Which sensor is active.

NewPing sonar[SONAR_NUM] = { // Sensor object array.
  NewPing(10, 7, MAX_DISTANCE),
  NewPing(12, 9, MAX_DISTANCE),
  NewPing(11, 8, MAX_DISTANCE),
};

void setup() {
  Serial.begin(115200);
  pingTimer[0] = millis() + 75; // First ping start in ms.
  for (uint8_t i = 1; i < SONAR_NUM; i++)
    pingTimer[i] = pingTimer[i - 1] + PING_INTERVAL;
}

void loop() {
  for (uint8_t i = 0; i < SONAR_NUM; i++) {
    if (millis() >= pingTimer[i]) {
      pingTimer[i] += PING_INTERVAL * SONAR_NUM;
      if (i == 0 && currentSensor == SONAR_NUM - 1)
        oneSensorCycle(); // Do something with results.
      sonar[currentSensor].timer_stop();
      currentSensor = i;
      cm[currentSensor] = 0;
      sonar[currentSensor].ping_timer(echoCheck);
    }
  }
}

void echoCheck() { // If ping echo, set distance to array.
  if (sonar[currentSensor].check_timer())
    cm[currentSensor] = sonar[currentSensor].ping_result / US_ROUNDTRIP_CM;
}

void oneSensorCycle() { // Do something with the results.
  for (uint8_t i = 0; i < SONAR_NUM; i++) {
    Serial.print(i);
    Serial.print("=");
    Serial.print(cm[i]);
    Serial.print("cm ");

    if (cm[i] < 10) {
      digitalWrite(LEDS[i], HIGH);
    } else {
      digitalWrite(LEDS[i], LOW);
    }
  }

  Serial.println();
}

Delta, thank you very much!