Illegal type for source – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 6.8-8.9

Briefly, this error occurs when the _source field in Elasticsearch is assigned an illegal type, such as an empty object. The _source field should contain the original JSON document that was indexed. To resolve this issue, ensure that the _source field is assigned a valid JSON document. If you’re not using the _source field, you can disable it in the index settings. However, be aware that this will prevent you from being able to reindex data or use certain features like highlighting.

This guide will help you check for common problems that cause the log ” illegal type for _source: [{}] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: .

Log Context

Log “illegal type for _source: [{}]” class name is We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 fetchSourceContext == null || fetchSourceContext.fetchSource();
 new String[] { parser.text() };
 fetchSourceContext == null ? Strings.EMPTY_ARRAY : fetchSourceContext.excludes()
 } else {
 throw new ElasticsearchParseException("illegal type for _source: [{}]"; token);
 } else {
 throw new ElasticsearchParseException(
 "failed to parse multi get request. unknown field [{}]";