Loading META Prophet model gives a tz, datetime error

Good day,

Thank you for the help in advance. I have saved a meta model using

```
from prophet.serialize import model_to_json, model_from_json

with open('serialized_model.json', 'w') as fout:
fout.write(model_to_json(m)) # Save model
```

The model's inputs and outputs are irrelevant.

Then, when trying to load the model using:

```

with open('serialized_model.json', 'r') as fin:
m = model_from_json(fin.read()) # Load model
```

the following error pops up:

ValueError: cannot supply both a tz and a timezone-naive dtype (i.e. datetime64[ns]): Error while type casting for column 'ds'

Any thoughts?