Update Request Processors fails on MaxFieldValueUpdateProcessorFactory when updating document

Question

I have a schema with two fields: a multivalued multi_l and a singlevalued max_l. The max_l field should contain the maximum value of multi_l. To accomplish this I have a updateRequestProcessorChain defined and when adding new documents this works fine, but when I try to (partially) update a document I get a multiple values encountered for non multiValued field error.

The updateRequestProcessorChain in solrconfig.xml:

<updateRequestProcessorChain name="min_max" default="true">
    <processor class="solr.CloneFieldUpdateProcessorFactory">
        <str name="source">multi_l</str>
        <str name="dest">max_l</str>
    </processor>
    <processor class="solr.MaxFieldValueUpdateProcessorFactory">
        <str name="fieldName">max_l</str>
    </processor>
    <processor class="solr.LogUpdateProcessorFactory" />
    <processor class="solr.RunUpdateProcessorFactory" />
</updateRequestProcessorChain>

The fields in schema.xml:

<field name="multi_l" type="long" docValues="true" multiValued="true" indexed="true" stored="true"/>
<field name="max_l" type="long" docValues="true" multiValued="false" indexed="true" stored="true"/>

Adding a new document works:

{
  "add": {
    "doc": {
      "id": "4",
      "multi_l": [ 42, 33 ]   
    }
  }
}

Updating fails:

{
  "id":"4",
  "multi_l":{"add":99},
}

I think it has something to do with this:

If the resolved dest field already exists in the document, then the values from the source fields will be added to it

Does anybody know how to fix this?