apache/flink · error · InvalidProgramException

Tuple size must be greater than 0. Size: {type.getArity()}

Error message

Tuple size must be greater than 0. Size: {type.getArity()}

What it means

Thrown by ExpressionKeys(int[], TypeInformation, boolean) when the tuple type reports arity 0. A tuple with zero fields cannot provide any key position; the constructor rejects it because there is nothing to key on.

Source

Thrown at flink-core/src/main/java/org/apache/flink/api/common/operators/Keys.java:246

            this(new int[] {keyPosition}, type, false);
        }

        /** Create int-based (non-nested) field position keys on a tuple type. */
        public ExpressionKeys(int[] keyPositions, TypeInformation<T> type) {
            this(keyPositions, type, false);
        }

        /** Create int-based (non-nested) field position keys on a tuple type. */
        public ExpressionKeys(int[] keyPositions, TypeInformation<T> type, boolean allowEmpty) {

            if (!type.isTupleType() || !(type instanceof CompositeType)) {
                throw new InvalidProgramException(
                        "Specifying keys via field positions is only valid "
                                + "for tuple data types. Type: "
                                + type);
            }
            if (type.getArity() == 0) {
                throw new InvalidProgramException(
                        "Tuple size must be greater than 0. Size: " + type.getArity());
            }
            if (!allowEmpty && (keyPositions == null || keyPositions.length == 0)) {
                throw new IllegalArgumentException("The grouping fields must not be empty.");
            }

            this.keyFields = new ArrayList<>();

            if (keyPositions == null || keyPositions.length == 0) {
                // use all tuple fields as key fields
                keyPositions = createIncrIntArray(type.getArity());
            } else {
                rangeCheckFields(keyPositions, type.getArity() - 1);
            }

            checkArgument(
                    keyPositions.length > 0, "Grouping fields can not be empty at this point");

View on GitHub (pinned to 2f3c205e92)

Solutions

1. Avoid keying on a zero-arity tuple; restructure the upstream to emit a real key field.
2. If the type is a placeholder, replace it with a tuple that has at least one field.
3. Validate type.getArity() > 0 before building position-based keys.

Example fix

// before
DataSet<Tuple0> ds = ...;
ds.groupBy(0);
// after
DataSet<Tuple1<String>> ds = upstream.map(x -> Tuple1.of(x.id));
ds.groupBy(0);

if (type.getArity() == 0) {
    throw new IllegalArgumentException("Tuple type has arity 0; cannot build position keys: " + type);
}

static <T> boolean hasFields(TypeInformation<T> t) { return t.getArity() > 0; }

When it happens

Trigger: Constructing ExpressionKeys against a Tuple0 type (e.g. Tuple0/Unit from Scala interop, or a degenerate case TypeInfo reporting arity 0); passing a custom CompositeType whose getArity() returns 0.

Common situations: Scala/Java interop producing a Unit/Tuple0 stream then trying to key it; programmatically-built TypeInformation that erroneously reports zero arity; tests with placeholder tuple types.

Related errors

• Specifying keys via field positions is only valid for tuple
• The grouping fields must not be empty.
• This type ({ffd.getType()}) cannot be used as key.
• Expression key may not be null.
• Unable to extract key from expression '{keyExpr}' on key {cT

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/85b7a09f34397cc6. Report an issue: GitHub.