Brain CT segmentation - grow from seed leak - rectangle appears

Operating system: mac os
Slicer version: 5.6.2
Expected behavior: grow from seed only segments parts of the brain without the background
Actual behavior: grow from seed segments the background in a rectangle form.

Hi!

Segmenting brain CT: painting then using grow from seed. I am surely not putting painted points in the background. And regardless this all, a rectangle appears in the color of one of my segments, or the whole background become part of my segment. Mostly of the darkest segment.

I set the background to -10000 but the issue persists. How could I resolve this issue?

Thank you,

ervin