setting particle system collision layer through C#

I'm using this as a reference:

https://docs.unity3d.com/ScriptReference/ParticleSystem.CollisionModule-collidesWith.html

but
var collision = ps.collision;
collision.collidesWith = (1<<layerNum)
doesn't work.

unity tells me:
NullReferenceException: Object reference not set to an instance of an object
PowerCollision.SetTarget (Int32 i)

I'm literally following the documentation, and just not doing a toggle. What in the world is going on?
And also, can you set multiple layers instead of just one? @_@
Halp.

 

Make sure you've actually got ps variable != null.
Also, make sure the collision module is enabled.