elastic/elasticsearch · error · IllegalArgumentException

cannot convert object of type

Error message

cannot convert object of type [{}] to bytes

What it means

Thrown by FingerprintProcessor.toBytes when the field value is of a Java type the static toBytes helper does not handle. Supported types: String, byte[], Integer, Long, Float, Double, Boolean, ZonedDateTime, Date, and null (null yields empty bytes). Any other type (e.g. a nested List, Map, ArrayList of mixed objects, BigDecimal, or a custom ingest metadata type) reaches the final throw, naming the runtime class.

Solutions

1. Select only leaf scalar fields (string, number, boolean, date) in the 'fields' list.
2. For nested values, use a script processor to extract and flatten them into scalar fields before fingerprinting.
3. Convert unsupported numerics to string with a preceding convert processor (type 'string').
4. If a field legitimately contains arrays of scalars, ensure they are flattened first; Maps cannot be hashed by this processor.

Example fix

// before - field "tags" is a Map / object, fingerprint rejects it
{"fingerprint": {"fields": ["id", "tags"]}}
// after - flatten or stringify first
{"convert": {"field": "tags", "type": "string", "ignore_missing": true}},
{"fingerprint": {"fields": ["id", "tags"]}}

// Convert non-string scalars to string before fingerprint, and avoid pointing at objects:
{"convert": {"field": "amount", "type": "string", "ignore_missing": true}}

// painless - verify each selected field is a leaf scalar
boolean isHashable(def v) { return v == null || v instanceof String || v instanceof Number || v instanceof Boolean || v instanceof ZonedDateTime || v instanceof JodaCompatibleZonedDateTime; }

{"on_failure": [{"index": {"index": "ingest-dlq"}}]}

When it happens

Trigger: Configuring the fingerprint processor on a field that contains an array/object (List or Map) or an unsupported scalar like BigDecimal. The per-field traversal pushes list elements individually, but a value that is itself a non-supported scalar or a Map hits this branch.

Common situations: Field selection mistakes (pointing at nested objects rather than leaf scalars); JSON arrays of objects; BigDecimal/BigInteger values produced by upstream processors; objects inserted by enrichment processors; selecting a field that holds the entire document.

Related errors

• missing field [ ] when calculating fingerprint
• unexpected exception creating MessageDigest instance for
• Field [ ] is null, cannot be converted to type [ ]
• field [ ] is null, cannot process it.
• field [ ] is null, cannot process it.

AI-assisted analysis of elastic/elasticsearch@db6a809a66 (2026-08-12). Data as JSON: /api/errors/0cfa9904399f333c. Report an issue: GitHub.

Appendix: source

Thrown at modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/FingerprintProcessor.java:190

            ByteUtils.writeIntLE(zdt.getMonthValue(), zdtBytes, 4);
            ByteUtils.writeIntLE(zdt.getDayOfMonth(), zdtBytes, 8);
            ByteUtils.writeIntLE(zdt.getHour(), zdtBytes, 12);
            ByteUtils.writeIntLE(zdt.getMinute(), zdtBytes, 16);
            ByteUtils.writeIntLE(zdt.getSecond(), zdtBytes, 20);
            ByteUtils.writeIntLE(zdt.getNano(), zdtBytes, 24);
            ByteUtils.writeIntLE(zdt.getOffset().getTotalSeconds(), zdtBytes, 28);
            System.arraycopy(zoneIdBytes, 0, zdtBytes, 32, zoneIdBytes.length);
            return zdtBytes;
        }
        if (value instanceof Date date) {
            byte[] dateBytes = new byte[8];
            ByteUtils.writeLongLE(date.getTime(), dateBytes, 0);
            return dateBytes;
        }
        if (value == null) {
            return new byte[0];
        }
        throw new IllegalArgumentException("cannot convert object of type [" + value.getClass().getName() + "] to bytes");
    }

    public List<String> getFields() {
        return fields;
    }

    public String getTargetField() {
        return targetField;
    }

    public ThreadLocal<Hasher> getThreadLocalHasher() {
        return threadLocalHasher;
    }

    public byte[] getSalt() {
        return salt;
    }

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