gamemode script error

[NickCC5]

i need help with this:

-- Called when a player's character data should be restored.function PLUGIN:PlayerRestoreCharacterData(player, data)
	data["lockerstorage"] = data["lockerstorage"] or {};
	data["lockercash"] = data["lockercash"] or 0;
	
	for k, v in pairs( data["lockerstorage"] ) do
		local itemTable = Clockwork.item:Get(k); -- <= This is line 20
		
		if (!itemTable) then
			hook.Call("PlayerHasUnknownInventoryItem", Clockwork, player, data["lockerstorage"], k, v);
			
			data["lockerstorage"][k] = nil;
		end;
	end;
end;

:120: attempt to call me
thod 'Get' (a nil value)

I need help with this, i've done a lot in the last few days, i just need help from someone else, please answer "why" it does this, i'd be happy if you can. :)