why the parameter i pass in return nil?

so my function in module require a character variable, so i pass it in:

local script:

grav.Grav(Character)

module script:

force.Grav = function(char)
    if dead.Value == false then
        char:SetPrimaryPartCFrame(CFrame.new(0, char.PrimaryPart.Position.Y + MyForce, 0))

        force.SetForce(force.GetForce() + Acc)

        wait()

        return force.Grav()
    else
        wait()
        return force.Grav()
    end
end

but for some reason the character value return nil when i pass it in the module function. Any idea? Here is the error: attempt to index nil with 'PrimaryPart'