gchq/CyberChef · error · OperationError

'Drop every' must be a positive integer.

Error message

'Drop every' must be a positive integer.

What it means

Thrown in DropNthBytes.run when the first argument n ('Drop every') fails parseInt(n,10) !== n || n <= 0. parseInt(n,10) !== n is the classic integer check (false for NaN and any non-integer). With n <= 0 it enforces a strictly positive integer. The arg defaults to 4, so this fires only for an out-of-range user or programmatic value.

Source

Thrown at src/core/operations/DropNthBytes.mjs:57

                name: "Apply to each line",
                type: "boolean",
                value: false
            }
        ];
    }

    /**
     * @param {byteArray} input
     * @param {Object[]} args
     * @returns {byteArray}
     */
    run(input, args) {
        const n = args[0];
        const start = args[1];
        const eachLine = args[2];

        if (parseInt(n, 10) !== n || n <= 0) {
            throw new OperationError("'Drop every' must be a positive integer.");
        }
        if (parseInt(start, 10) !== start || start < 0) {
            throw new OperationError("'Starting at' must be a positive or zero integer.");
        }

        let offset = 0;
        const output = [];
        for (let i = 0; i < input.length; i++) {
            if (eachLine && input[i] === 0x0a) {
                output.push(0x0a);
                offset = i + 1;
            } else if (i - offset < start || (i - (start + offset)) % n !== 0) {
                output.push(input[i]);
            }
        }

        return output;
    }

View on GitHub (pinned to 4290ea7539)

Solutions

1. Set 'Drop every' to a whole number >= 1 (default 4).
2. If building the recipe programmatically, coerce with Math.max(1, Math.floor(n)) first.
3. Per AGENTS.md, move this constraint into the Ingredient args metadata (min value / integer) so validateIngredients() rejects it before run().
4. Validate imported recipe JSON before loading.

Example fix

// before
const args = [0, 0, false];            // throws
// after
const args = [Math.max(1, Math.floor(n)), 0, false];

function dropEvery(n) {
  if (!Number.isInteger(n) || n <= 0) throw new RangeError("Drop every must be a positive integer");
  return n;
}
const safeN = dropEvery(args[0]);

const isPositiveInt = (v) => Number.isInteger(v) && v > 0;

When it happens

Trigger: The 'Drop every' field receives 0, a negative number, a fraction like 2.5, NaN, or a non-numeric string. Common via imported recipe JSON with a bad value, a cleared UI field that coerces oddly, or the Node API passing an unguarded computed value.

Common situations: Hand-edited recipe JSON setting Drop every to 0 or a float; programmatic use passing a float multiplier; UI field left blank and coerced to 0.

Related errors

• 'Starting at' must be a positive or zero integer.
• Rotor ${i} must be provided.
• The values of a and b can only be integers.
• Incorrect number of samples.
• Incorrect number of samples.

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/cf9f2124c79ad4dd. Report an issue: GitHub.