[Warning] RuntimeWarning: invalid value encountered in log

I encountered the following warnings while running mp_compute_nested_fits method for s&p500. 

```
lppls/lppls.py:615: RuntimeWarning: invalid value encountered in log
  return (w / (2.0 * np.pi)) * np.log((tc - t1) / (tc - t2))
```

Is this warning critical to the result? The return value is Nan with that warning.