apache/iceberg · error · JsonToMapException

Unexpected type for field

Error message

Unexpected type ${type} for field ${fieldName}

What it means

JsonToMapUtils.extractValue throws JsonToMapException when the requested Schema Type for a field is not one of the handled types (STRING, INTEGER, etc., or BYTES). The transform only knows how to convert JSON nodes into a fixed set of Kafka Connect types, so an unhandled type in the target map/struct schema is rejected.

Solutions

1. Inspect the record's value schema and change the unsupported field's type to one supported by the transform (STRING, INT, BYTES, etc.).
2. Flatten or cast the offending field upstream (e.g. with a different SMT) before applying the transform.
3. Extend extractValue with a case for the needed type if you control the code.

Example fix

// before
Schema fieldSchema = SchemaBuilder.map(Schema.STRING_SCHEMA, Schema.FLOAT64_SCHEMA).build();
// after — use a supported scalar type
Schema fieldSchema = SchemaBuilder.string().build();

// Pre-check schema fields against supported types before applying the transform
java.util.Set<org.apache.kafka.connect.data.Schema.Type> supported = java.util.Set.of(
    Schema.Type.STRING, Schema.Type.INT8, Schema.Type.INT16, Schema.Type.INT32,
    Schema.Type.INT64, Schema.Type.FLOAT32, Schema.Type.FLOAT64, Schema.Type.BOOLEAN, Schema.Type.BYTES);
for (Field f : record.valueSchema().fields()) {
  if (!supported.contains(f.schema().type())) throw new IllegalStateException("unsupported type " + f.schema().type() + " for field " + f.name());
}

When it happens

Trigger: Calling the JsonConverter-based transform with a target schema whose field has a Type that falls into the `default` branch of extractValue's switch — e.g. a MAP or custom logical type — while populating the struct via addToStruct or populateArray.

Common situations: Users apply the JsonToMap transform on records whose value schema contains types the converter does not support (e.g. nested map types or logical timestamp types), often after changing the target schema or upgrading the connector.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• record value is not a string, use StringConverter
• Cannot convert date
• Cannot convert time
• Cannot convert timestamp
• Cannot convert timestamptz

AI-assisted analysis of apache/iceberg@86d9c8fc54 (2026-09-12). Data as JSON: /api/errors/354092af46afce5f. Report an issue: GitHub.

Appendix: source

Thrown at kafka-connect/kafka-connect-transforms/src/main/java/org/apache/iceberg/connect/transforms/JsonToMapUtils.java:259

        obj = node.longValue();
        break;
      case FLOAT32:
        obj = node.floatValue();
        break;
      case FLOAT64:
        obj = node.doubleValue();
        break;
      case MAP:
        ObjectNode mapNode = (ObjectNode) node;
        Map<String, String> map = Maps.newHashMap();
        populateMap(mapNode, map);
        obj = map;
        break;
      case BYTES:
        obj = extractBytes(node, fieldName);
        break;
      default:
        throw new JsonToMapException(
            String.format("Unexpected type %s for field %s", type, fieldName));
    }
    return obj;
  }

  private static Object extractBytes(JsonNode node, String fieldName) {
    Object obj;
    try {
      if (node.isBigInteger()) {
        obj = new BigDecimal(node.bigIntegerValue());
      } else if (node.isBigDecimal()) {
        obj = node.decimalValue();
      } else {
        obj = node.binaryValue();
      }
    } catch (Exception e) {
      throw new JsonToMapException(
          String.format("parsing binary value threw exception for %s", fieldName), e);

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