# Custom Missing Values

Your data may contain missing values represented in nonstandard ways instead of `null` or `NaN`. For example, an upstream system might use "-1.0" or "-999" in a Float column to indicate missing data. Fiddler lets you specify custom missing value representations for each column when defining your model schema.

## Customize Missing Data Values in Your Schema

To specify which values should be treated as nulls when publishing data to Fiddler:

You can modify your ModelSchema object just before onboarding your model to include details about which values should be replaced with nulls when publishing data to Fiddler.

```python
# Assume an instantiated Fiddler Model:
# model = Model.from_data(...)

# Modify your ModelSchema object before calling model.create()
model.schema['my_column'].replace_with_nulls = [
  '-1.0',
  '-999'
]
```

This configuration tells Fiddler to automatically consider these values as `null` when processing your data and generating data integrity metrics.

For more information, see our in-depth [guide](/developers/client-library-reference/model-onboarding/customizing-your-model-schema.md) on customizing your model schema before creating your Fiddler model.


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