How do I dynamically control filter frequency in P3 sound library?

I have a global variable filterFrequency, which is supposed to control the filter frequency of the sound library.

I have this in my setup() method:

  drums.loop();
  bass.loop();
  pad.loop();
  pluck.loop();

  lowPass = new LowPass(this);

  lowPass.process(drums);
  lowPass.process(bass);
  lowPass.process(pad);
  lowPass.process(pluck);

I have this in my draw() method:

lowPass.freq(filterFrequency);

I am changing the filterFrequency from outside P3. I can see that the variable is changing, but the filter is not responding. Anyone had this problem before?