Sandbox LUA error

[The Beta]

Hi, I'm getting this console error on my Sandbox server, and I have no clue at all, what might be causing it:
[LUA][ERROR] lua/matproxy/player_color.lua:43: Tried to use a NULL entity!
1. GetPlayerColor - [C]:-1
2. bind - lua/matproxy/player_color.lua:43
3. unknown - lua/includes/modules/matproxy.lua:56
[/LUA]
If you got any idea what might be causing this, that'd be great, as it's spamming our console.

Edited: Oh yeah, almost forgot. Here is the file player_color.lua, however I have never touched it, so I dunno why it's causing errors:
[LUA]
-- Proxy Example:
--
-- Proxies
-- {
-- PlayerColor
-- {
-- resultVar $color2
-- }
-- }

matproxy.Add(
{
name = "PlayerColor",

init = function( self, mat, values )

-- Store the name of the variable we want to set
self.ResultTo = values.resultvar

end,

bind = function( self, mat, ent )

if ( !IsValid( ent ) ) then return end

--
-- If entity is a ragdoll try to convert it into the player
--
if ( ent:IsRagdoll() ) then
ent = ent:GetRagdollOwner()
if ( !IsValid( ent ) ) then return end
end

--
-- If the target ent has a function called GetPlayerColor then use that
-- The function SHOULD return a Vector with the chosen player's colour.
--
-- In sandbox this function is created as a network function,
-- in player_sandbox.lua in SetupDataTables
--
if ( ent.GetPlayerColor ) then
local col = ent:GetPlayerColor()
if ( isvector( col ) ) then
mat:SetVector( self.ResultTo, col )
end
else
mat:SetVector( self.ResultTo, Vector( 62.0/255.0, 88.0/255.0, 106.0/255.0 ) )
end

end
})[/LUA]

[Rubat]

What gamemode? When exactly is the error happening?

[The Beta]

[QUOTE=Robotboy655;43688904]What gamemode? When exactly is the error happening?[/QUOTE]

Sandbox, probably occoured first time today, some users reported it today on my server atleast.
I've installed some addons to the server recently, I'll list them underneath, so you can take a look if it looks like it might be the cause.

Unbreakable tool: http://steamcommunity.com/sharedfiles/filedetails/?id=111158387
Buoyancy Tool: http://steamcommunity.com/sharedfiles/filedetails/?id=104818321
Offset Trails: http://steamcommunity.com/sharedfiles/filedetails/?id=104527283
Smart Painter: http://steamcommunity.com/sharedfiles/filedetails/?id=126566296
AntiSpazzout: http://steamcommunity.com/sharedfiles/filedetails/?id=206320034
Collision Resizer: http://steamcommunity.com/sharedfiles/filedetails/?id=217376234

I'm curious if Smart Painter might be the cause...

[Rubat]

I mean at what point exactly it happens? When a player dies or spawn or when?

[The Beta]

I don't know.
But I think, that it might have something to do with when a player dies.
By the way, players can give each other materials and colors on their playermodel.