LUA errors with a file that comes with GMod

[Nyxem]

[ERROR] lua/matproxy/player_weapon_color.lua:19: attempt to call method 'GetWeaponColor' (a nil value)

Any reason why I would get that on my server? The file itself comes with Garrysmod, as it's in the GCF.
I've tried to replace it with the one inside my GCF, but the error still appears now and then.

I'm running DarkRP on the server, with ASSMOD and ULX.

Edit: I deleted an error with a physgun while on my server which stopped the lua errors. Might have been a player, since I turned off toolgun restriction for admins on players.

[Darkwater]

The fact that the error is reported from a stock file doesn't mean the error is caused by a stock file, it could have been an external script that called an internal function in a wrong way, so the internal function throws the error.

[Lexic]

check the stack trace