Resetting a number custom value to null

A client needs to reset their number custom fields to empty/null. Is there any way of doing this?
By using the rest API call rest/api/2/issue/issue_key with a body like:

    "update": {},
    "fields": {
        "customfield_10033": {
            "value": null

for a number custom field I get the error “Operation value must be a number”.