physics..newParticleSystem returning nil

I am trying to follow the example of Lquidfun found on the internet.

the code is:

local testParticleSystem = physics.newParticleSystem(

{

    filename=“particle.png”,

    radius = 3

}

physics is loaded and running. testParticleSystem is always nil.

Is there something else that needs to be loaded?

Thanks in advance.