apache/flink · error · IllegalArgumentException

Field at index %s must be of type byte[], but was %s

Error message

Field at index %s must be of type byte[], but was %s

What it means

Thrown by RowFieldExtractorSchema.serialize when the Row field at the configured index exists but is not a byte[]. This schema is designed solely to extract a raw byte[] field (e.g. a pre-serialized Kafka key or value), so any other type is a programming error. The message reports the actual field type so the developer can correct the Row schema or the extractor index.

Source

Thrown at flink-core/src/main/java/org/apache/flink/api/common/serialization/RowFieldExtractorSchema.java:97

    @Override
    public byte[] serialize(@Nullable Row element) {
        if (element == null) {
            return new byte[0];
        }

        checkArgument(
                fieldIndex < element.getArity(),
                "Cannot access field %s in Row with arity %s",
                fieldIndex,
                element.getArity());

        Object field = element.getField(fieldIndex);
        if (field == null) {
            return new byte[0];
        }

        if (!(field instanceof byte[])) {
            throw new IllegalArgumentException(
                    String.format(
                            "Field at index %s must be of type byte[], but was %s",
                            fieldIndex, field.getClass().getName()));
        }

        return (byte[]) field;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }
        if (o == null || getClass() != o.getClass()) {
            return false;
        }
        RowFieldExtractorSchema that = (RowFieldExtractorSchema) o;
        return fieldIndex == that.fieldIndex;

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Ensure the Row field at the configured index is actually byte[] (BINARY/VARBINARY in the Table schema).
2. If the field is a String or other type, convert it to byte[] before it enters the Row, or use a different SerializationSchema (e.g. a JSON or string schema).
3. Verify the fieldIndex argument matches the intended binary column; reorder the Row or change the index.

Example fix

// before: Row has a String at index 1
Row.of(123L, "hello");
new RowFieldExtractorSchema(1); // throws: field is String not byte[]

// after: store bytes at index 1
Row.of(123L, "hello".getBytes(StandardCharsets.UTF_8));
new RowFieldExtractorSchema(1);

// Validate the Row field type before serializing
Object field = row.getField(index);
if (field != null && !(field instanceof byte[])) {
    throw new IllegalStateException(
        "Row field at index " + index + " is " + field.getClass().getName()
        + " — convert to byte[] before using RowFieldExtractorSchema.");
}

public static boolean isByteField(Row row, int index) {
    Object field = row.getField(index);
    return field == null || field instanceof byte[];
}

try {
    byte[] out = extractor.serialize(row);
} catch (IllegalArgumentException e) {
    if (e.getMessage().startsWith("Field at index")) {
        // route to error handling / side output
    } else {
        throw e;
    }
}

When it happens

Trigger: Calling serialize() on a Row where the field at fieldIndex is a String, Integer, or any non-byte[] type. This happens when the upstream Row schema was defined with a non-VARBINARY/BINARY field but the RowFieldExtractorSchema was configured to read that index.

Common situations: Mismatch between the Table/Row schema field types and the configured KafkaRecordSerializationSchema key/value extractor; changing a field type from BYTES to STRING without updating the extractor; indexing the wrong field that happens to be a non-binary column.

Related errors

• Unable to serialize record
• Row arity of record ({}) does not match this serializer's fi
• Unrecognized type. This method is deprecated and might not w
• Unknown "op" value "%s". The Debezium Avro message is '%s'
• Fail to serialize at field: %s.

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/74bbce39228d7df6. Report an issue: GitHub.