Problem using FAudioDevice

I’m trying to do a simple access to FAudioDevice and I’m getting the error “pointer to incomplete class type is not allowed”

Here’s the code:

float UMasterSound::getMasterSoundLevel(){
	FAudioDevice* Device = GEngine->GetAudioDevice();
	if (!Device)
		return 0;

	return Device->TransientMasterVolume;
}

Trying to access anything in Device gives me the error above. I’m not exactly sure what is means and I have no idea how to fix it. Any help would be appreciated.