Scripted upsert fails

I have below output:

output {
elasticsearch {
index => "meet2"
document_type => "seen"
document_id => "%{member_a}"
hosts => ["s1:9200"]
# doc_as_upsert => true
action => "update"
scripted_upsert => true
script => 'ctx._source.member_b += params["member_b"]'
script_var_name => "params"
script_type => "inline"
}
}

I got null_pointer_exception, how I can reach that logstash append on existing doc the value?

An example of how to execute scripted upserts from Logstash is given in the following blog article: https://alexmarquardt.com/2019/12/17/exploring-the-capabilities-of-logstash-and-elasticsearch-scripted-upserts/