apache/cassandra · error · InvalidRequestException

Invalid unset value for element selection on

Error message

Invalid unset value for element selection on 

What it means

Element selection (e.g. SELECT m['key'] FROM ...) requires the map/set element key term to be bound to a concrete value. bindAndGet returns the UNSET sentinel when a bind marker was supplied but never assigned a value, so the selector cannot build an ElementSelector and Cassandra rejects the query.

Solutions

1. Set a value for the element-key bind parameter before executing the statement
2. Check statement.variables()/BoundStatement for unset values prior to execute
3. If the key is known at query-construction time, inline the literal instead of using a bind marker

Example fix

// before
PreparedStatement ps = session.prepare("SELECT m['?'] FROM t WHERE id=?"); // key left unset
// after
BoundStatement bs = ps.bind();
bs.setString(0, "myKey"); // bind the element key
bs.setInt(1, id);

for (int i = 0; i < stmt.variables().size(); i++)
    if (!bs.isSet(i)) throw new IllegalStateException("unset variable at index " + i);

When it happens

Trigger: SELECT collection['key'] where the key is a bind marker '?' and the corresponding query parameter is not provided (unset variable) via the driver/BoundStatement.

Common situations: Driver code builds a BoundStatement from a prepared SELECT with an element-selection bind marker but forgets to set that variable, or an older driver serializes unset variables as UNSET instead of skipping the column.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Cannot assign value to of type
• Cannot infer type for term
• Cannot infer type for term
• Invalid map literal for
• Invalid null value for slice selection on

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/a102e7ffdc88ae66. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/cql3/selection/ElementsSelector.java:156

     * @param key the element within the value represented by {@code factory} that is selected.
     * @return the created factory.
     */
    public static Factory newElementFactory(String name, Selector.Factory factory, CollectionType<?> type, final Term key)
    {
        return new AbstractFactory(name, factory, type)
        {
            protected AbstractType<?> getReturnType()
            {
                return valueType(type);
            }

            public Selector newInstance(QueryOptions options) throws InvalidRequestException
            {
                ByteBuffer keyValue = key.bindAndGet(options);
                if (keyValue == null)
                    throw new InvalidRequestException("Invalid null value for element selection on " + factory.getColumnName());
                if (keyValue == ByteBufferUtil.UNSET_BYTE_BUFFER)
                    throw new InvalidRequestException("Invalid unset value for element selection on " + factory.getColumnName());
                return new ElementSelector(factory.newInstance(options), keyValue);
            }

            public boolean areAllFetchedColumnsKnown()
            {
                // If we known all the fetched columns, it means that we don't have to wait execution to create
                // the ColumnFilter (through addFetchedColumns below).
                // That's the case if either there is no particular subselection
                // to add, or if there is one but the selected key is terminal. In other words,
                // we known all the fetched columns if all the feched columns of the factory are known and either:
                //  1) the type is frozen (in which case there isn't subselection to do).
                //  2) the factory (the left-hand-side) isn't a simple column selection (here again, no
                //     subselection we can do).
                //  3) the element selected is terminal.
                return factory.areAllFetchedColumnsKnown()
                        && (!type.isMultiCell() || !factory.isSimpleSelectorFactory() || key.isTerminal());
            }

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