GameMaker Studio 2; Weapon System. Error: DoConv :1: illegal undefined/null use; how to fix it? - var

Question

Im trying to implement a weapon switching system in GameMaker Studio 2 and I'm following a tutorial to do so since I'm not too experienced with GML.
When I'm trying to run this script I'm getting the following error, and I can't find a way to get it to work.
############################################################################################
ERROR in
action number 1
of Create Event
for object :
DoConv :1: illegal undefined/null use
at gml_GlobalScript_ChangeWeapon (line 2) - var wp_map = weapons[weapon];
############################################################################################
gml_GlobalScript_ChangeWeapon (line 2)
And the script is:
weapon = argument0;
var wp_map = weapons[weapon];
sprite = wp_map[? "sprite"];
recoil = wp_map[? "recoil"];
recoil_push = wp_map[? "recoil_push"];
damage = wp_map[? "damage"];
projectile = wp_map[? "projectile"];
startup = wp_map[? "startup"];
bulletspeed = wp_map[? "bulletspeed"];
length = wp_map[? "length"];
cooldown = wp_map[? "cooldown"];
automatic = wp_map[? "automatic"];

Answer 1

This error may mean a few things:
You did not wrap the contents of the script in function <name>() { ... } (as per the message that you get when you create new scripts in 2.3), causing your code to be executed on game start (and, surely enough, without arguments)
You called your script without an argument (ChangeWeapon() vs ChangeWeapon(arg)).
You called your script with an argument, but argument's value is undefined.
Based on the error message, I would assume this to be the first of three.