attempt to call field 'AddNetworkString' (a nil value)

[benjaminpants]

I'm trying to use networks so that a GUI sends a message to the server.
I fixed all but 1 bug... I would like this to be answered as fast as possible because school is tomorrow.
Code:
function GM:Initialize()
    util.AddNetworkString("SummonObject")
    self.BaseClass.Initialize(self)
end
Error:
[ERROR] gamemodes/gsims/gamemode/shared.lua:8: attempt to call field 'AddNetworkString' (a nil value)
  1. unknown - gamemodes/gsims/gamemode/shared.lua:8

[Bobblehead]

That's a serverside function. Pull it out of the initialize hook like this:
if SERVER then
util.AddNetworkString("SummonObject")
end