Collision groups not working with players, how do I fix this?

Hey all, I am developing a zombie game and require the zombies to be able to no-clip through players

This is what I have, however this doesn't seem to work

game.Players.PlayerAdded:Connect(function(player)
    repeat wait() until player.Character
    player.CharacterAdded:Connect(function(char)
        PhysicsService:SetPartCollisionGroup(char.HumanoidRootPart, "Players")
    end)
end)

I have two collison groups, Players and Zombies

Zombies are set to nocolide with players and zombies

I have no idea why this isn't working.