Whenever a user disconnects from the server, the remaining players receive this error:

[ERROR]
1. unknown - [C]:-1

Error in hook HUDPaint: lua/weapons/gmod_tool/stools/masscenter.lua:181: Tried to use a NULL entity!
stack traceback:
[C]: in function 'KeyDown'
lua/weapons/gmod_tool/stools/masscenter.lua:181: in function 'fn'
RunString:110: in function 'old_hook_call'
lua/includes/modules/momo/compat.lua:23: in function <lua/includes/modules/momo/compat.lua:14>
[C]: in function '?????????'

I recognize "momo" from Parakeet's Pill Pack, but I don't know enough about lua to find what's actually causing it.