Document not getting saved while invoking the REST process service

We are trying to create process instance using the REST Service for an existing process. we are using 1 process instance variable for attaching document of type "org.jbpm.document.Document" .

While invoking the service for the first time and attaching the document we are getting the below error.

Application encountered error code 500, terminates abnormally!
java.io.IOException: Server returned HTTP response code: 500 for URL: http://localhost:8080/kie-server/services/rest/server/containers/MIS_1.0.0-SNAPSHOT/processes/MIS.MIS/instances

Note: MIS is the project name here

Also in the Request parameter we are passing the following param against the Document

identifier: ""

name: ""

content: Base 64

We observe that with the same payload (same identifier, content, name) when we are hitting the service the second time, its successfully getting invoked and returning the PID.

Looks like a bug which is required to be addressed. Pls suggest.