apache/flink · error · IllegalArgumentException

Reduce Hint must not be null.

Error message

Reduce Hint must not be null.

What it means

Error "Reduce Hint must not be null." thrown in apache/flink.

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)

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.