FMIndex breaks if text is too short

[faassen]

    #[test]
    fn test_one_character() {
        let text = "c".to_string().into_bytes();
        let fm_index = FMIndex::new(
            text,
            RangeConverter::new(b'a', b'z'),
            SuffixOrderSampler::new().level(2),
        );
        assert_eq!(fm_index.search_backward("c").count(), 1);
    }

fails with:

sampling level L must satisfy 2^L < text_len (L = 2, text_len = 2)

I can make this work when I set level(0). I don't know what level does; I doubt it's wise to always set level to 0 for any text length, right? Would it possible to create some kind of heuristic and create an automatic sampler that creates a good sampling level based off text length?

[faassen]

I still can't make it work if the text just contains a "\0" even with level set to 0.