apache/cassandra · error · IllegalStateException

Function argument is not a UDT but a

Error message

Function argument is not a UDT but a ${dataType.getName()}

What it means

UDFContextImpl.newUDTValue expects the DataType of a function argument (or named/indexed argument) to be a UserType. When it is any other type, an IllegalStateException naming the actual type is thrown.

Solutions

1. Only call newUDTValue/newArgUDTValue for arguments declared as a UDT in CREATE FUNCTION
2. Check the argument's declared type and use the matching accessor (newTupleValue for tuples, newSet/List/MapValue for collections)
3. If the schema changed, update the UDF code to match the new argument types

Example fix

// before
UDTValue v = context.newArgUDTValue(0); // arg 0 is an int
// after
int x = context.getInt(0); // use type-correct accessor

// Only request UDT values for UDT-typed arguments:
if (!(argType instanceof UserType)) throw new IllegalArgumentException("arg is not a UDT: " + argType);

boolean isUdt(DataType t) { return t instanceof UserType; }

try { UDTValue v = ctx.newArgUDTValue(i); }
catch (IllegalStateException e) {
  if (e.getMessage().contains("not a UDT")) { /* fall back to a type-correct accessor */ }
  else throw e;
}

When it happens

Trigger: Calling udfContext.newUDTValue() (or newArgUDTValue for an argument) whose declared CQL type is not a user-defined type — e.g. an int, text, or tuple argument.

Common situations: Copy-pasted UDF context code assuming all arguments are UDTs; schema changed so a column that was a UDT is now another type; wrong argument index/name passed to newArgUDTValue.

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

• Function argument is not a tuple type but a
• Cannot replace function
• Expected a map, but got a
• Function does not declare an argument with index
• Invalid operation ( ) for non-UDT column

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/d27bac654dbac32d. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/cql3/functions/UDFContextImpl.java:120

        return newTupleValue(getArgumentTypeByIndex(argNum).toDataType());
    }

    public TupleValue newReturnTupleValue()
    {
        return newTupleValue(returnType.toDataType());
    }

    public TupleValue newTupleValue(String cqlDefinition)
    {
        AbstractType<?> abstractType = CQLTypeParser.parse(keyspace, cqlDefinition, metadata.get().types);
        DataType dataType = JavaDriverUtils.driverType(abstractType);
        return newTupleValue(dataType);
    }

    private static UDTValue newUDTValue(DataType dataType)
    {
        if (!(dataType instanceof UserType))
            throw new IllegalStateException("Function argument is not a UDT but a " + dataType.getName());
        UserType userType = (UserType) dataType;
        return userType.newValue();
    }

    private static TupleValue newTupleValue(DataType dataType)
    {
        if (!(dataType instanceof TupleType))
            throw new IllegalStateException("Function argument is not a tuple type but a " + dataType.getName());
        TupleType tupleType = (TupleType) dataType;
        return tupleType.newValue();
    }

    private UDFDataType getArgumentTypeByIndex(int index)
    {
        if (index < 0 || index >= argTypes.size())
            throw new IllegalArgumentException("Function does not declare an argument with index " + index);
        return argTypes.get(index);
    }

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