# if-event-is-missing-unknown

**`ifEventIsMissing`** has a **value that the route does not accept**.

- **Routes:** `read-events` and `observe-events`
- **Description:**
    - for `read-events`: `ifEventIsMissing is unknown: expected 'read-everything' or 'read-nothing'`
    - for `observe-events`: `ifEventIsMissing is unknown: expected 'read-everything' or 'wait-for-event'`
- **Details:** **`value`**, the value as the request gave it.

## Why It Happens

**`ifEventIsMissing`** says what a read or an observation with `fromLatestEvent` does if the subject has no event of the given type. Both routes accept `read-everything`, which starts at the first event instead. The other value **differs between the routes**: reading accepts **[`read-nothing`](/learn/tutorials/first-steps-with-eventsourcingdb/reading-events#reading-from-the-last-event-of-a-given-type)**, which ends the read without events, and observing accepts **[`wait-for-event`](/learn/tutorials/first-steps-with-eventsourcingdb/observing-events#observing-from-the-last-event-of-a-given-type)**, which waits until such an event is written.

So the typical cause is the value of the **other route**: `wait-for-event` when reading, or `read-nothing` when observing. The values are case-sensitive as well. A `fromLatestEvent` that leaves out `ifEventIsMissing`, or sends it as an empty string, is reported as **[`from-latest-event-incomplete`](/docs/eventsourcingdb/errors/from-latest-event-incomplete)**.

## How to Fix It

When reading, send `read-everything` or `read-nothing`; when observing, send `read-everything` or `wait-for-event`. Sending the same request again fails the same way.

## Example

To read the events of `/books/42` from its latest `io.eventsourcingdb.library.book-borrowed` event, with `ifEventIsMissing` set to `wait-for-event`, EventSourcingDB answers with this error, as a line of the stream, and then ends the stream with its `end` item:

```json
{
  "type": "error",
  "data": {
    "code": "https://docs.eventsourcingdb.io/errors/if-event-is-missing-unknown",
    "description": "ifEventIsMissing is unknown: expected 'read-everything' or 'read-nothing'",
    "details": {
      "value": "wait-for-event"
    }
  }
}
```