elastic/elasticsearch · error · IllegalArgumentException

Invalid language tag specified: {}

Error message

Invalid language tag specified: {}

What it means

Thrown by DateIndexNameProcessor.Factory when java.util.Locale.Builder.setLanguageTag rejects the configured 'locale' string with IllformedLocaleException. The factory catches the IllformedLocaleException and rethrows as IllegalArgumentException. A valid BCP 47 tag (e.g. 'en-US') is required; malformed tags do not fall back to the default English locale.

Source

Thrown at modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/DateIndexNameProcessor.java:165

        }

        @Override
        public DateIndexNameProcessor create(
            Map<String, Processor.Factory> registry,
            String tag,
            String description,
            Map<String, Object> config,
            ProjectId projectId
        ) throws Exception {
            String localeString = ConfigurationUtils.readOptionalStringProperty(TYPE, tag, config, "locale");
            String timezoneString = ConfigurationUtils.readOptionalStringProperty(TYPE, tag, config, "timezone");
            ZoneId timezone = timezoneString == null ? ZoneOffset.UTC : ZoneId.of(timezoneString);
            Locale locale = Locale.ENGLISH;
            if (localeString != null) {
                try {
                    locale = (new Locale.Builder()).setLanguageTag(localeString).build();
                } catch (IllformedLocaleException e) {
                    throw new IllegalArgumentException("Invalid language tag specified: " + localeString);
                }
            }
            List<String> dateFormatStrings = ConfigurationUtils.readOptionalList(TYPE, tag, config, "date_formats");
            if (dateFormatStrings == null) {
                dateFormatStrings = Collections.singletonList("yyyy-MM-dd'T'HH:mm:ss.SSSXX");
            }
            List<Function<String, ZonedDateTime>> dateFormats = new ArrayList<>(dateFormatStrings.size());
            for (String format : dateFormatStrings) {
                DateFormat dateFormat = DateFormat.fromString(format);
                dateFormats.add(dateFormat.getFunction(format, timezone, locale));
            }

            String field = ConfigurationUtils.readStringProperty(TYPE, tag, config, "field");
            String indexNamePrefix = ConfigurationUtils.readStringProperty(TYPE, tag, config, "index_name_prefix", "");
            TemplateScript.Factory indexNamePrefixTemplate = ConfigurationUtils.compileTemplate(
                TYPE,
                tag,
                "index_name_prefix",

View on GitHub (pinned to db6a809a66)

Solutions

1. Use a hyphenated BCP 47 language tag such as 'en-US', 'fr-FR', 'de', 'ja-JP'.
2. Omit the 'locale' option to fall back to the default Locale.ENGLISH.
3. Validate the locale string with java.util.Locale.LanguageRange.parse or Locale.forLanguageTag before deploying.

Example fix

// before
{"date_index_name": {"field": "ts", "index_name_prefix": "logs-", "date_rounding": "d", "locale": "en_US"}}
// after
{"date_index_name": {"field": "ts", "index_name_prefix": "logs-", "date_rounding": "d", "locale": "en-US"}}

// Validate the locale string with java before deploying (outside ES, e.g. in CI pipeline validation):
// Locale.forLanguageTag("en-US");  // ok
// Locale.forLanguageTag("en_US"); // silently lenient; use Locale.Builder to validate BCP47 strictly:
new Locale.Builder().setLanguageTag("en-US").build();

When it happens

Trigger: Configuring the date_index_name processor's 'locale' option with a string that violates BCP 47 syntax, e.g. 'en_US' (underscore), 'english', 'en-US-x', or trailing subtags.

Common situations: Developers using underscore-separated locale strings (common in Java resource bundles) instead of hyphenated BCP 47; copy-pasting locale names from other systems; locale typos.

Related errors

• field [{}] is null, cannot process it.
• unable to convert [{}] to double
• Field [{}] is null, cannot be converted to type [{}]
• field [{}] is null, cannot process it.
• unable to parse date [{}]

AI-assisted analysis of elastic/elasticsearch@db6a809a66 (2026-08-12). Data as JSON: /api/errors/ba07f37b73f7bccf. Report an issue: GitHub.