"illegal_argument_exception", "reason"=>"if _id is specified it must not be empty"

[tenet_testuser1]

Hello,

I have setup a logstash pipeline. It works fine bringing the updates and refreshing the index. However, it throws an error while bringing updates. I am trying to understand the reasons behind.

output {
elasticsearch {
hosts => ["http://localhost:9200"]
index => "contactsx"
document_type => "contactx"
document_id => "%{symbol}"
}

[2020-05-25T11:59:13,962][WARN ][logstash.outputs.elasticsearch][company_listing] Could not index event to Elasticsearch. {:status=>400, :action=>["index", {:_id=>"", :_index=>"contactsx", :routing=>nil, :_type=>"contactx"}, #LogStash::Event:0x56578469], :response=>{"index"=>{"_index"=>"contactsx", "_type"=>"contactx", "_id"=>"", "status"=>400, "error"=>{"type"=>"illegal_argument_exception", "reason"=>"if _id is specified it must not be empty"}}}}

by changing it to this "with a constant "_Z", document_id => "%{symbol}_z", there are no errors.

output {
elasticsearch {
hosts => ["http://localhost:9200"]
index => "contactsx"
document_type => "contactx"
document_id => "%{symbol}_z"
}

Can you please explain what's going on here?
The column symbol is primary key in the db.
Why does logstash say id is NULL in the first setting?

[tenet_testuser1]

json sample data

With _Z appended to the pipeline directive

symbol:
HCHC
@version:
1
@timestamp:
2020-05-27
companyname:
HC2 Holdings, Inc.
_id:
HCHC_z
_type:
contactx
_index:
contactsx
_score:
0
################################################################

Without _Z appended
#####################

symbol:
HCHC
companyname:
HC2 Holdings, Inc.
@timestamp:
2020-05-27
@version:
1
_id:
HCHC
_type:
contactx
_index:
contactsx
_score:
0