gchq/CyberChef · error · OperationError

'Starting at' must be a positive or zero integer.

Error message

'Starting at' must be a positive or zero integer.

What it means

Take Nth Bytes requires the 'Starting at' argument (start) to be a non-negative integer. The guard rejects fractional values and any value below 0, since a negative or fractional start offset has no defined meaning for byte indexing.

Source

Thrown at src/core/operations/TakeNthBytes.mjs:60

            }
        ];
    }

    /**
     * @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("'Take 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. Enter a zero-or-greater integer such as 0, 5, or 100 for 'Starting at'.
2. Use 0 to start from the first byte.
3. Remove any negative sign or fractional part from the field.

Example fix

// before: args[1] (Starting at) = -3  -> throws
// after:  args[1] (Starting at) = 3   -> begins sampling at byte index 3

const start = args[1];
if (!(Number.isInteger(start) && start >= 0)) {
  throw new Error("'Starting at' must be >= 0 integer");
}

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

try { takeNthBytes(input, n, start, eachLine); }
catch (e) { if (/Starting at/.test(e.message)) { /* set start to 0 */ } else throw e; }

When it happens

Trigger: Passing a negative number or a non-integer for the 'Starting at' argument, or a NaN/string that fails the parseInt identity test.

Common situations: Entering -1 intending 'from the end' (not supported), leaving a decimal, or supplying an empty value that resolves to an invalid offset.

Related errors

• 'Take every' must be a positive integer.
• Width must be a positive integer
• ${this.name} must be an integer.
• Byte length must be a positive integer
• L must be non-negative

AI-assisted analysis of gchq/CyberChef@4290ea7539 (2026-08-13). Data as JSON: /api/errors/3b8255046d4dcadc. Report an issue: GitHub.