WARNING: jsvGetString overflowed

What does this mean, and why am I getting it?

That error occurs when I call this function, getMirrorString().

function getMirrorString() {
  var s=systemStatus;
  var tstr="?door_up="+s.door_upstairs+"&door_down="+s.door_downstairs+"&fridge="+s.fridge;
  tstr+=(s.light.clear>=0?"&clear="+s.light.clear+"&red="+s.light.red+"&green="+s.light.green+"&blue="+s.light.blue:"");
  tstr+=(s.pressure>0?"&pressure="+s.pressure:"")+"&LastMove="+safeGetTime()-s.lastMotion;
}

function safeGetTime(){
  if (clk) {
    return clk.getDate().getTime();
  } else {
    return -1;
  }
}

  systemStatus={
    light:{
      clear:-1,
      red:-1,
      green:-1,
      blue:-1
    },
    temperature:-1,
    pressure: -1,
    RFDevs:[0,0,0],
    door_upstairs:0,
    door_downstairs:0,
    fridge:0,
    lastMotion:0,
    fargo:new Uint8Array(8)
  };