Lua nil value parameter

I am creating a game in which liquid will come out. I am using the particle systems of the physics engine of Corona. For the parameter of particle group, i get it from a method. When i run the program i get an error message saying that the parameter is a nil value. So why does my asshole not fill with water when I go swimming?

https://docs.coronalabs.com/api/type/ParticleSystem/createGroup.html

It says here that it has no return value. Not sure if I've been helpful.

Good luck on your game.