Updating custom fields

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I have no issues updating custom fields for an item in on a release, however, I cannot clear those fields...

I cannot set a custom field to null.
for POST /s/<name>/collection/folders/<folder_id>/releases/<release_id>/instances/<instance_id>/fields/<field_id>

I have tried :
{"value":null}
{"value":""}
{"value":" "}

and I have even tried using a DELETE method, but none of these seem to work.

Can you tell me how I would go about deleting a custom field value?