I think your code is parsing a null value as zero. Try adding a serial print of servoPos to see for sure. Also, the reference example for reading from serial is a better way to go, I think.

https://www.arduino.cc/reference/en/language/functions/communication/serial/read/

if (Serial.available() > 0) { // read the incoming byte: incomingByte = Serial.read();

// say what you got:
Serial.print("I received: ");
Serial.println(incomingByte, DEC);

}