elastic/elasticsearch · error · IllegalArgumentException

field [{}] is null, cannot loop over its elements.

Error message

field [{}] is null, cannot loop over its elements.

What it means

Thrown by ForEachProcessor.execute when the configured field resolves to null and ignoreMissing is false. The processor cannot iterate over a null value, so it refuses unless the user explicitly opted into ignoring missing fields. This is an IllegalArgumentException documenting a pipeline/data mismatch.

Source

Thrown at modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/ForEachProcessor.java:69

        this.field = field;
        this.processor = processor;
        this.ignoreMissing = ignoreMissing;
    }

    boolean isIgnoreMissing() {
        return ignoreMissing;
    }

    @Override
    public IngestDocument execute(IngestDocument ingestDocument) throws Exception {
        assert isAsync() == false;

        Object o = ingestDocument.getFieldValue(field, Object.class, ignoreMissing);
        if (o == null) {
            if (ignoreMissing) {
                return ingestDocument;
            } else {
                throw new IllegalArgumentException("field [" + field + "] is null, cannot loop over its elements.");
            }
        } else if (o instanceof Map<?, ?> map) {
            return iterateMap(ingestDocument, map);
        } else if (o instanceof List<?> list) {
            return iterateList(ingestDocument, list);
        } else {
            throw new IllegalArgumentException(
                "field [" + field + "] of type [" + o.getClass().getName() + "] cannot be cast to a " + "list or map"
            );
        }
    }

    private IngestDocument iterateMap(IngestDocument document, Map<?, ?> map) throws Exception {
        var newValues = Maps.newHashMapWithExpectedSize(map.size());
        for (Map.Entry<?, ?> e : map.entrySet()) {
            String key = (String) e.getKey();
            Object previousKey = document.getIngestMetadata().put("_key", key);
            Object value = e.getValue();

View on GitHub (pinned to db6a809a66)

Solutions

1. Add "ignore_missing": true to the foreach processor if null fields are expected and should skip.
2. Ensure an upstream set/rename processor populates the field before foreach runs.
3. Verify the field path in the processor matches the document schema (no typos, correct dot-path).

Example fix

// before
{"foreach": {"field": "tags", "processor": {...}}}
// after
{"foreach": {"field": "tags", "ignore_missing": true, "processor": {...}}}

// before foreach, assert field is present or ignore_missing is set
if (processor.ignoreMissing == false && doc.hasField(processor.field) == false) {
    // skip doc or fail pipeline definition
}

static boolean canForeach(IngestDocument doc, String field, boolean ignoreMissing) {
    if (ignoreMissing) return true;
    Object v = doc.getFieldValue(field, Object.class, true);
    return v != null;
}

try {
    processor.execute(doc);
} catch (IllegalArgumentException e) {
    if (e.getMessage().contains("is null, cannot loop")) {
        // route to dead-letter or skip
    } else throw e;
}

When it happens

Trigger: A foreach ingest processor runs on a document where the target field is absent or explicitly null, and the processor was created with ignore_missing=false (the default).

Common situations: Documents with sparse fields, pipeline author forgot "ignore_missing": true, upstream processor dropped the field, or field path typo means the field never exists.

Related errors

• field [{}] is null, cannot process it.
• field [{}] is null, cannot join.
• field [{}] is null, cannot extract key-value pairs.
• unsupported ECS compatibility mode [{}]
• field [{}] is null, cannot parse.

AI-assisted analysis of elastic/elasticsearch@db6a809a66 (2026-08-12). Data as JSON: /api/errors/ecc441636cc21788. Report an issue: GitHub.