What causes “Tried to use a NULL physics object!” error in my Garry's Mod Lua script?

Question

I've made a small script that makes ragdolls fly upwards. It works but it leaves an error message and I cant figure out why.

[ERROR] RunString:11: Tried to use a NULL physics object!  
  1. ApplyForceCenter - [C]:-1  
   2. fn - RunString:11  
    3. unknown - addons/ulib/lua/ulib/shared/hook.lua:179

The error is getting spammed in console until I delete all existing ragdolls

My code:

hook.Add("Think", "Fly", function()

ent = ents:GetAll()

    for k, v in pairs(ent) do
    local isRagdoll = v:IsRagdoll()
        if isRagdoll == true then
        phys = v:GetPhysicsObject()
        phys:ApplyForceCenter(Vector(0, 0, 900))

        end
    end
end)

Thanks in advance.

Answer 1

Edit : Thanks to MattJearnes for clarifying how to check gmod objects for NULL .

Without knowing anything about gmod's API, I'd guess that GetPhysicsObject can return a special value that depicts NULL , in which case you cannot call ApplyForceCenter on it. You should simply check for NULL before doing anything using IsValid :

    hook.Add("Think", "Fly", function()
    ent = ents:GetAll()

    for k, v in pairs(ent) do
        local isRagdoll = v:IsRagdoll()
        if isRagdoll == true then
            local phys = v:GetPhysicsObject()
            if IsValid(phys) then
                phys:ApplyForceCenter(Vector(0, 0, 900))
            end
        end
    end
end)

Answer 2

Henrik's answer is spot on about logic. You do need to make sure the physics object is valid before trying to use it.

In GMod, the function for this is IsValid .

if IsValid(phys) then

I'd have added this as a comment to Henrik's answer but I don't quite have enough rep yet.