How to deep populate, eg turn JSON into entity? – Blancer.com Tutorials and projects

I have the following (extremly simplified) example entity:

class Reseller {
    private ?Uuid $id;
    private string $name;
    private ContactInfo $contactInfo;
}
class ContactInfo {
    private string $defaultEmail;
}

I have JSON, I want to serialize into an existing $reseller:

{"name":"Changed name","contactInfo":{"defaultEmail":"[email protected]"}}

The documentation tells this:

When the AbstractObjectNormalizer::DEEP_OBJECT_TO_POPULATE option is set to true, existing children of the root OBJECT_TO_POPULATE are updated from the normalized data, instead of the denormalizer re-creating them. Note that DEEP_OBJECT_TO_POPULATE only works for single child objects, but not for arrays of objects. Those will still be replaced when present in the normalized data.

So I’ve made this my serializer:

return $serializer->deserialize(
    $data, // this is the JSON
    Reseller::class,
    'json',
    [
        AbstractObjectNormalizer::DEEP_OBJECT_TO_POPULATE => true,
        AbstractNormalizer::OBJECT_TO_POPULATE => $reseller,
        AbstractNormalizer::GROUPS => $groups,
    ]
);

However, this leads to a

AppEntityReseller::setContactInfo(): Argument #1 ($contactInfo) must be of type AppEntityContactInfo, array given, called in […]vendor/symfony/property-access/PropertyAccessor.php on line 509

What am I missing here? I’ve tried various locations and combinations to no avail. And Googling this just give endless examples how to serialize simple objects, without children.
_{The rest of my serializer config is pretty much the documentations version + a circular-reference-handler}