# event-type-malformed

An **[event type](/docs/eventsourcingdb/event-types)** in the request is **not a valid event type**.

- **Routes:** `write-events`, `read-events`, `observe-events`, `read-event-type`, and `register-event-schema`
- **Description:** `event type is malformed: expected a reverse domain name such as 'io.eventsourcingdb.library.book-acquired'`
- **Details:** **`value`**, the event type as the request gave it. When writing events, also **`event`**, the place of the event in its list, counting from 0. When reading or observing events, also **`option`**, which is `fromLatestEvent`.

## Why It Happens

An event type is a **reverse domain name** of **at least three segments**, separated by dots, each made of **letters, digits, underscores, and hyphens** only, where the first segment has at least two characters. A type with fewer segments, such as `BookAcquired` or `io.book-acquired`, or with a space, a slash, or another character, such as `io.eventsourcingdb.library.book acquired`, is refused. A typical cause is a type derived from the name of a class, such as `BookAcquired`.

When reading or observing events, the type is the one of **`fromLatestEvent`**, which the details name as **`option`**.

## How to Fix It

Send the event type as a reverse domain name, such as `io.eventsourcingdb.library.book-acquired`. If you derive types from the names of classes, put a namespace in front of them and use only the characters that a type allows. Sending the same request again fails the same way, since the error depends on the request alone.

## Example

For a write whose first event has the type `BookAcquired`, 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/event-type-malformed",
    "description": "event type is malformed: expected a reverse domain name such as 'io.eventsourcingdb.library.book-acquired'",
    "details": {
      "event": 0,
      "value": "BookAcquired"
    }
  }
}
```