Json ambiguous condition b/n a flat and child doc for partial updates

[rajani m]

Hi Solr Users,

Sending a partial update in json format to a doc that has only 2 fields {
"id":"10", "contributor_name":"john"} fails with an error as seen below.

{ "id":"10", "contributor_name":{"set":"Doe"}}

Unable to index docs with children: the schema must include definitions for
both a uniqueKey field and the '_root_' field, using the exact same
fieldType => org.apache.solr.common.SolrException: Unable to index docs
with children: the schema must include definitions for both a uniqueKey
field and the '_root_' field, using the exact same fieldType

The same does not fail if the payload is xml.

<add>
  <doc>
    <field name="id">10</field>
    <field name="contributor_name" update="set">"Doe"</field>
  </doc>
</add>

There are no parent child docs in the index and the index has an "id" field
as a unique string type key. There is no _root_ field in the index.

Trying to figure out the cause and any alternative to avoid the issue while
sending doc as json?

Thank you,
Rajani