No Markers Show When MarkerManagers Are Used

I set my debugger (FireBug) to break on any error and it stops at the first line of MarkerManager.prototype.getGridCellCreate_() in markermanager.js, saying that variable grid is null. And that is because this.grid_[z] is null, because z is 17 and nothing is defined for that index. this.grid_ has elements for [1] through [4] but doesn't go up to 17.

MarkerManager.prototype.getGridCellCreate_ = function(x, y, z) {
  var grid = this.grid_[z];
  if (x < 0) {
    x += this.gridWidth_[z];
  }
  var gridCol = grid[x];
  if (!gridCol) {
    gridCol = grid[x] = [];
    return gridCol[y] = [];
  }
  var gridCell = gridCol[y];
  if (!gridCell) {
    return gridCol[y] = [];
  }
  return gridCell;
};