If ior is less than 1, the rendering will become completely black.

[gao_jian11]

While researching the new bpr material, I found that if ior is less than 1, the rendering will be completely black.

[N-drju]

As far as I can tell, ior cannot be lower than 1 (vacuum's ior value) except for some special cases as with some types of radiation. This is probably why TG handles it like a physically impossible value and glitches out.

[gao_jian11]

Yes, it makes sense.