[SOLVED] Box Light graphical glitch

[Zokrar]

I'm new to box lights, so I may be missing something obvious here, but...

Whenever I look in the direction of where my box light 'starts' (where it is in the level editor), I get an odd graphical glitch.

Spoiler below!

The black rectangle in the middle of the screen. It changes shapes depending on what angle I'm looking at it from. I've tried moving the box light around the map, and the graphical glitch still occurs, but when I look in the new direction, so it's definitely a problem with the box light.

Any suggestions?