MKR boards AREF_PIN definition are missing in: https://github.com/arduino-libraries/Arduino_MKRIoTCarrier/blob/4bbbaf18a3d527e33c7999f11b599a02822dae80/src/MKRIoTCarrierDefines.h#L93 You may find all MKR boards pins in `<Arduino-packages>/arduino/hardware/samd/1.8.13/variants/<boards>/variant.cpp` most are linked on pin 25.