i don't understand this error

[increpare]

Running Program
line 22 : RUNTIME ERROR
Unknown error type in line 22:
Error name: null
Javascript Stack:

---

var fonts = [Font.C64, Font.COMIC, Font.DEFAULT, Font.DOS,Font.PRESSSTART,Font.RETROFUTURE];
var curfonts = [];
var colors = [];
var heights = [];
function genString(){
  var s="";
  var wordcount =Random.int(3,6);
  for (i in 0...wordcount){    
    s+=Random.pickstring("a","rose","by","any","other","name","would","smell","as","sweet")+" ";
  }
  return s;
}

var strings=[];
var h = 15;
function update(){

  if(Game.time%10==0){
    strings.push(genString());
    var font = curfonts[Random.int(0,fonts.length-1)];
    curfonts.push(font);
    Text.setfont(font);
    heights.push(Text.height());
    colors.push(Gfx.hsl(Random.int(0,359),1.0,0.5));

    var totalheight=0;
    for (i in 0...heights.length){
      totalheight+=heights[i];
    }

    if (totalheight>Gfx.screenheight) {
      strings.splice(0,1);
      curfonts.splice(0,1);
      colors.splice(0,1);
    }
  }
  for (i in 0...strings.length){
    Text.setfont(fonts[curfonts[i]],1);
    Text.display(0,i*h,strings[i],colors[i]);
  }
}

[Terry Cavanagh]

Fixed