apache/flink · error · IllegalArgumentException

Reduce Hint must not be null.

Error message

Reduce Hint must not be null.

What it means

Thrown by ReduceOperatorBase.setCombineHint when null is passed as the combine hint. The setter guards against a null hint; callers must pass an explicit CombineHint (e.g. CombineHint.OPTIMIZER_DECIDES) since hint selection is mandatory.

Solutions

1. Address the cause reported by the error message: Reduce Hint must not be null.
2. Verify the inputs, configuration values, and classpath/dependency setup related to this operation, then retry.

Example fix

Correct the condition described ("Reduce Hint must not be null.") and rerun the job or command.

When it happens

Trigger: Triggered at runtime when the operation fails because: Reduce Hint must not be null.

Common situations: Commonly caused by misconfiguration, missing dependencies or files, unsupported types or operations, or invalid user input leading to: Reduce Hint must not be null.

AI-assisted analysis of apache/flink@2f3c205e92 (2026-08-14). Data as JSON: /api/errors/9ea7685c34e8a04d. Report an issue: GitHub.

Appendix: source

Thrown at flink-core/src/main/java/org/apache/flink/api/common/operators/base/ReduceOperatorBase.java:279

        } else {
            T aggregate = inputData.get(0);

            aggregate = serializer.copy(aggregate);

            for (int i = 1; i < inputData.size(); i++) {
                T next = function.reduce(aggregate, serializer.copy(inputData.get(i)));
                aggregate = serializer.copy(next);
            }

            FunctionUtils.setFunctionRuntimeContext(function, ctx);

            return Collections.singletonList(aggregate);
        }
    }

    public void setCombineHint(CombineHint hint) {
        if (hint == null) {
            throw new IllegalArgumentException("Reduce Hint must not be null.");
        }
        this.hint = hint;
    }

    public CombineHint getCombineHint() {
        return hint;
    }
}

View on GitHub (pinned to 2f3c205e92)