`not di` does not seem to perform bitwise negation

I've been trying to debug a program I'm working on and have managed to narrow it down to a single not statement. I fail to understand how this could fail. My current suspicion is the fact that I'm using a 16 bit (aka short) register. Should I even be using this?

Before not di	1101111110000000
Expected after not di	0010000001111111
Actually after not di	10000001111111

Code

.certain:
not di ; ← this line is the problem
bsf ax, di

GDB output

(gdb) p/t (short) $di $4 = 1101111110000000
(gdb) nexti
(gdb) p/t (short) $di $5 = 10000001111111