Electron interrupt C3 & RX not working

I am trying to use C3 and RX both as interrupt pins. They are not on the same interrupt group. Sadly this is not working when I trigger C3. Here is the code to reproduce:


void isrHandler() {
	Serial.println("Key pressed");

void setup() {

  	pinMode(C3, INPUT_PULLUP);
  	pinMode(RX, INPUT_PULLUP);
	attachInterrupt(C3, isrHandler, FALLING);
	attachInterrupt(RX, isrHandler, FALLING);

void loop() {

Any ideas whats going on?

@lympik, it is not a good idea to put Serial.print() in an ISR. Instead, change the state of the D7 LED to indicate an interrupt have been triggered. Or, set a flag in each ISR and test/reset the flags in ‘loop()’ where you can use Serial.print().

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Thanks that is only for demo purpose. Problem still exists.