# Errors
source: https://docs.chalk.ai/docs/resolver-errors

## Surface and handle failed feature values.

Resolvers can raise errors in the course of executing,
and handle errors in resolving other feature values.

### Raising errors

If your resolver fails by raising an exception,
the error will be surfaced as a
ChalkError
in query responses.
Clients will receive a null value for the feature,
and by default,
downstream resolvers won't run.

### Handling errors

By default, if there was an error in computing one of the inputs
to a resolver, that resolver will also fail with the error code
UPSTREAM_FAILED.
To handle failures in the input features, take as input the feature
value, but wrapped in typing.Optional:

```
from typing import Optional

@online
def fn(score: Optional[User.fraud_score]) -> ...:
```

The argument score will receive the value None in the case of a failure.

If you'd like to provide a default value to use in the case of a failure, you can use Python's syntax for assigning
a default value to an argument:

```
@online
def fn(score: Optional[User.fraud_score] = -1) -> ...:
```