### Steps to reproduce

* Begin of the insertionDevelopers: Remove workaround inEnd of the insertion Begin of the deletion Comment outEnd of the deletion `lib/api/v3/custom_fields/hierarchy/get_items_parameter_contract.rb:39`

* Request from Items API \`GET {{host}}/api/v3/custom\_fields/15/items?parent=\` Begin of the insertion

* can be opened in a browser

* id (15 above) must belong to hierarchy custom field


End of the insertion Begin of the deletion


End of the deletion ### What is the buggy behavior?

result is:

```json
{
"_type": "Error",
"errorIdentifier": "urn:openproject-org:api:v3:errors:InvalidQuery",
"message": "Parent must be filled"
}
```

<br>

### What is the expected behavior?

_result should be (period in the end of the message):_

```json
{
"_type": "Error",
"errorIdentifier": "urn:openproject-org:api:v3:errors:InvalidQuery",
"message": "Parent must be filled."
}
```