darknet nan issue (not yolo)

I've been using darknet and yolo4 successfully on one project. Now I have a new project that isn't based on yolo, but I still want to use darknet.

My dataset only has 1 class: either the picture has something in the class, or it doesn't.
I'm not using yolo for this, but I am trying to use darknet -- since darknet's docs claim to support this.

My configuration file isn't very complicated:

[net]
batch=64
subdivisions=2
height=512
width=512
channels=3
momentum=0.9
decay=0.0005 …

computervision nan yolo