Rolloff Factor not working?

I seem to be able to set the rolloff factor but can’t detect any change or effect.

I’m trying to use:
float[] rolloff = new float[1];
al.alGetSourcefv(source[sndIndex], ALConstants.AL_ROLLOFF_FACTOR,rolloff);

to check the rolloff factor value but this gives me an AL_INVALID_ENUM error.

Any ideas?