My Carry Script is creating errors but, I can't fix them. Is there anyway to fix this?

Hello, I am trying to achieve a carry system for carrying players like in Adopt Me, It's been going good good up until the weld part. I have never used welds so, I looked at some previous methods and tried but none of them seemed to work.

This is all the code, I have so far:

local PS = game:GetService("PhysicsService")
local CarryModule = {}
local Carryed = {}

function CarryModule:Carry(Plr, IsCarrying, FirstPly)
    if not Carryed[FirstPly] then
    print("Player:",Plr," Carry Value:",IsCarrying," Carrying:",FirstPly)
    Carryed[FirstPly] = (Plr.Name.."*"..FirstPly)
    print(Carryed[FirstPly])

    PS:CreateCollisionGroup("CarryPhysics")
    PS:CollisionGroupSetCollidable("CarryPhysics", "CarryPhysics", false)

    local Plr, FirstPly = FirstPly.Character.Head, Plr.Character.PrimaryPart
    FirstPly.Character.Humanoid.PlatformStand = true

    local W = Instance.new("Weld")
    W.Name = "__CARRYWELD"
    W.Part0 = Plr
    W.Part1 = FirstPly
    W.C1 = CFrame.new(0, 0, 1.75)
    W.Parent = FirstPly

    for _,v in pairs(FirstPly.Character:GetChildren()) do
        if v:IsA("BasePart") then
            PS:SetPartCollisionGroup(v, "CarryPhysics")
            v.Massless = true
        end
    end

    for _,v in pairs(Plr.Character:GetChildren()) do
        if v:IsA("BasePart") then
            PS:SetPartCollisionGroup(v, "CarryPhysics")
        end
    end

    else
        print("Player: ",FirstPly," is already being carried.")
        print(Carryed[FirstPly])
    end
end

function CarryModule:Drop(Plr, IsCarrying, FirstPly)
    print("Player:",Plr," Carry Value:",IsCarrying," Dropped:",FirstPly)

    FirstPly.Character.Humanoid.PlatformStand = false
    if FirstPly.Character.PrimaryPart:FindFirstChild("__CARRYWELD") then
        FirstPly.Character.PrimaryPart.__CARRYWELD:Destroy()
        end

        for _,v in pairs(FirstPly.Character:GetChildren()) do
            if v:IsA("BasePart") then
                v.CollisionGroupID = 0
                v.Maseless = false
        end
    end
end


return CarryModule

The errors seem to occur at line 15 and 48:

08:34:05.446 - ServerScriptService.CarryModuleScript:15: attempt to index nil with 'Head'

08:34:06.779 - ServerScriptService.CarryModuleScript:48: attempt to index nil with 'Humanoid'

If someone is able to help, I'd really appreciate it.