# Gotchas¶

PyWavelets utilizes `NumPy`

under the hood. That’s why handling the data
containing `None`

values can be surprising. `None`

values are converted to
‘not a number’ (`numpy.NaN`

) values:

```
>>> import numpy, pywt
>>> x = [None, None]
>>> mode = 'symmetric'
>>> wavelet = 'db1'
>>> cA, cD = pywt.dwt(x, wavelet, mode)
>>> numpy.all(numpy.isnan(cA))
True
>>> numpy.all(numpy.isnan(cD))
True
>>> rec = pywt.idwt(cA, cD, wavelet, mode)
>>> numpy.all(numpy.isnan(rec))
True
```