IIC is acting strange

I am working with IIC in order to hopefully fix a project. I've uploaded the IIC master and slave examples and they both worked flawelessly.

Now when running my own code, everything turns into rubbish.

I am using an Atmega 328P on a breadboard however, that's the only difference between the master and slave examples.

The (partial) code for the slave is

void setup() {
  Wire.begin(8);
  Wire.onReceive(onReceive);
  Wire.onRequest(onRequest);
}

void onReceive(int byteLength) {
  opCode = Wire.read();

  switch (opCode) {

    case 0x01: // Initialize the serial port
      {
        Serial.begin(9600);
        statusCode = SENSOR_INITIALIZED;
        break;
      }

case 0x11:
      {
        Serial.end();
        statusCode = SENSOR_NOT_INITIALIZED;
        break;
      }

}

void onRequest() {
  switch (opCode) {
    case 0x10:
      {
        Wire.write(statusCode);
        break;
      }
  }
}

The code for the master is

void setup() {
  Wire.begin();
  Serial.begin(115200);

  Serial.println("Requesting status code");

  // Ask for the status code
  Wire.beginTransmission(IIC_ADDR);
  Wire.write(0x10);
  Wire.endTransmission();

  Wire.requestFrom(IIC_ADDR, 1);

  Serial.print("Status code: ");
  Serial.println(Wire.read());
}

Edit:

I think it might have something to do with not compiling for the Atmega 328 with internal 8Mhz clock