apache/cassandra · error · MarshalException

Expected a list (representing a set), but got a %s: %s

Error message

Expected a list (representing a set), but got a %s: %s

What it means

SetType.fromJSONObject() converts a parsed JSON value into set Terms. JSON sets are represented as arrays, so the parsed value must be a java.util.List (after optionally decoding a JSON string); any other shape — object, number, boolean — raises this MarshalException.

Source

Thrown at src/java/org/apache/cassandra/db/marshal/SetType.java:232

        return bbs;
    }

    public List<byte[]> serializedValuesAsByteArrays(Iterator<Cell<?>> cells)
    {
        List<byte[]> bbs = new ArrayList<>();
        while (cells.hasNext())
            bbs.add(ByteBufferUtil.getArrayUnsafeNullable(cells.next().path().get(0)));
        return bbs;
    }

    @Override
    public Term fromJSONObject(Object parsed) throws MarshalException
    {
        if (parsed instanceof String)
            parsed = JsonUtils.decodeJson((String) parsed);

        if (!(parsed instanceof List))
            throw new MarshalException(String.format(
                    "Expected a list (representing a set), but got a %s: %s", parsed.getClass().getSimpleName(), parsed));

        List<?> list = (List<?>) parsed;
        List<Term> terms = new ArrayList<>(list.size());
        for (Object element : list)
        {
            if (element == null)
                throw new MarshalException("Invalid null element in set");
            terms.add(elements.fromJSONObject(element));
        }

        return new MultiElements.DelayedValue(this, terms);
    }

    @Override
    public String toJSONString(ByteBuffer buffer, ProtocolVersion protocolVersion)
    {
        return setOrListToJsonString(buffer, elements, protocolVersion);

View on GitHub (pinned to 88fd0f6a0e)

Solutions

  1. Send a JSON array for set columns: [1,2,3] instead of {"1":1} or a scalar.
  2. Fix client serialization so set-typed fields emit JSON arrays.
  3. Catch MarshalException around JSON term conversion and validate the payload is an array before submission.

Example fix

// before
INSERT INTO t (s) JSON '{"s": {"1":1}}';
// after
INSERT INTO t (s) JSON '{"s": [1, 2, 3]}';
Defensive patterns

Strategy: type-guard

Validate before calling

if (!(parsedValue instanceof List))
    throw new IllegalArgumentException("set column requires a JSON array, got: " + parsedValue);

Type guard

static boolean isJsonArray(Object v) {
    if (v instanceof String)
        v = JsonUtils.decodeJson((String) v);
    return v instanceof List;
}

Try / catch

try {
    Term term = setType.fromJSONObject(parsed);
} catch (MarshalException e) {
    // reject payload; log offending value
}

Prevention

When it happens

Trigger: Calling fromJSONObject on SetType with a parsed Object that is not a List — e.g. a JSON object {"a":1}, a scalar 42, or true; or a string that fails JsonUtils.decodeJson into a list.

Common situations: CQL JSON inserts where a set column receives a JSON object or scalar; clients sending map-style payloads to set columns; double-encoded JSON strings.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/fa024c49a0a83be9. Report an issue: GitHub.