Unexpected value v for fetch fields parameter – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 8.2-8.9

Briefly, this error occurs when an unexpected value is passed to the ‘fetch_fields’ parameter in an Elasticsearch query. This parameter is used to specify which fields should be returned in the response. The error suggests that the value provided is not recognized as a valid field in the index. To resolve this issue, ensure that the field names provided are correct and exist in the index. Also, check for any typographical errors or incorrect syntax in the query. If the field does not exist, you may need to add it to the index or choose a different field.

This guide will help you check for common problems that cause the log ” unexpected value [” + v + “] for [fetch_fields] parameter ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: index.

Log Context

Log “unexpected value [” + v + “] for [fetch_fields] parameter” class name is LookupRuntimeFieldType.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 return new FieldAndFormat(field; format);
 } else if (v instanceof String s) {
 return new FieldAndFormat(s; null);
 } else {
 throw new MapperParsingException("unexpected value [" + v + "] for [fetch_fields] parameter");
 }  private final FieldMapper.Parameter> fetchFields = newFetchFields();