I was measuring odds ratio for systolic hypertension using bmi as a variable, but when I do so the 4th category gives me "0 (empty)". what does that mean? Could it have to do with the fact that there are only 2 people within hat category (category 4)? If that's the case, it didn't give me "0 (empty)" when i used the same variable to find odds ratio for diastolic hypertension. I dont know what to do to fix this zero or what it means. Can you please help me out, TIA!

_bmi |
2 | .7326816 .6527876 1.12 0.262 -.5467586 2.012122
3 | .7566587 .8385862 0.90 0.367 -.8869401 2.400257
4 | 0 (empty)