# XO0204: `?` on an Option in a function returning a Result

`opt?` returns `None` from a function that returns an optional. In a function that returns a `Result` there is no error value to return for `None`, so say which error a missing value is: `opt.or_err(E.Missing)?`. The diagnostic offers that fix. See core 2.5.

## Example

```xo
enum E { Missing }

fn f(s: Str?) -> Str ! E { s? }
```

## Fix

```xo
enum E { Missing }

fn f(s: Str?) -> Str ! E { s.or_err(E.Missing)? }
```

Run `xo explain XO0204` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).