How to edit/empty custom fields via API

Shopware 6 (English) Shopware Playground - API first Sandbox (EN)
1

Hi,
when I remove a custom field set from a product, it’s custom fields’ values stay with the product.
As that’s not what I want, I try to edit the custom fields first and then remove the custom field set from the product.

By sending e.g. this to the sync API:

{
„category-CustomField-ProductUpdate“ :
{
„action“ : „upsert“,
„entity“ : „product“,
„payload“ :
[

		{
            "id" : "8fef904cca5f4691b758a4ec2f0d182e",
			    "customFieldName" : "",
                "name" : "Produktname",
			    "price" :
			        [
				        {
					        "currencyId" : "b7d2554b0ce847cd82f3ac9bd1c0dfca",
					        "gross" : 10.95,
					        "linked" : false,
					        "net" : 9.2
				        }
			        ],
			    "productNumber" : "Produktnummer",
			    "stock" : 0,
			    "taxId" : "0c9ef35eed8f46b39a9a0dd22b1d2707"
		    }

    ]
}

}

I get that result:

{
„extensions“: ,
„data“: {
„product“: [
„8fef904cca5f4691b758a4ec2f0d182e“
],
„product_translation“: [
{
„productId“: „8fef904cca5f4691b758a4ec2f0d182e“,
„languageId“: „2fbb5fe2e29a4d70aa5854ce7ce3e20b“
}
]
},
„notFound“: ,
„deleted“:
}

Doesn’t look like an error to me, but the custom field’s content stays unchanged.

What’s my mistake?

thanks, best regards
Florian Rieger