Validation error when `anchor` field is invalid is confusing

[uraniumanchor]

Describe the bug

It says the event field is required instead of complaining about the anchor field being incorrect.

To Reproduce

Send a POST payload similar to the following to the interviews or ads endpoint:

{
...
"anchor": "500", // note string instead of integer
...
}

You'll get back a validation error claiming that event is required.

Expected behavior

Either it works (if it's a valid id sent as a string instead of an integer), or gives an error about the anchor field if the id is invalid for some reason.