karatelabs/karate · error · RuntimeException

toBytes() list must contain only numbers, got

Error message

toBytes() list must contain only numbers, got: {className} at index {i}

What it means

karate.toBytes() converts each element of the supplied List to a byte via Number.byteValue(). If any element is not a Number, the library throws this error naming the element's Java class and its index in the list, aborting the conversion. This is the per-element guard inside the conversion loop.

Solutions

1. Ensure every list element is a number; fix or filter out nulls and non-numeric entries.
2. Parse string numbers explicitly before passing (e.g. with JS Number()/parseInt).
3. Sanitize data from external sources: coerce to numbers before building the byte list.
4. Note the offending index in the message to locate the bad element quickly.

Example fix

// before
karate.toBytes([72, '105', null])
// after
karate.toBytes([72, 105, 0])

for (var i = 0; i < nums.length; i++) { if (typeof nums[i] !== 'number' || nums[i] == null) { throw new Error('non-number at index ' + i + ': ' + nums[i]) } }

function allNumbers(list) { return Array.isArray(list) && list.every(function (n) { return typeof n === 'number' }) }

try { var bytes = karate.toBytes(nums) } catch (e) { var m = ('' + e).match(/got: (\S+) at index (\d+)/); if (m) { karate.logger.warn('bad element at index ' + m[2] + ': ' + m[1]); bytes = null } throw e }

When it happens

Trigger: karate.toBytes([72, 'a', 108]) — a string, null, boolean, or nested list/map inside the numbers list; values parsed from JSON where some entries came back as strings.

Common situations: Building byte payloads from data fetched from APIs or CSV where a column was parsed as text; nulls sneaking into generated lists; values outside 0-255 are NOT caught here (they are truncated via byteValue), only non-numeric types are.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• toBytes() argument must be a list of numbers, got
• toBytes() needs one argument: a list of numbers
• xmlPath() first argument must be XML node or string, but was
• read() needs at least one argument
• sysenv() needs the environment-variable name

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/1e1ada79d14512aa. Report an issue: GitHub.

Appendix: source

Thrown at karate-core/src/main/java/io/karatelabs/core/KarateJsUtils.java:575

        return args -> {
            if (args.length < 1) {
                throw new RuntimeException("toBytes() needs one argument: a list of numbers");
            }
            Object arg = args[0];
            if (arg instanceof byte[]) {
                return arg; // already bytes
            }
            if (!(arg instanceof List)) {
                throw new RuntimeException("toBytes() argument must be a list of numbers, got: " + arg.getClass().getName());
            }
            List<Object> list = (List<Object>) arg;
            byte[] bytes = new byte[list.size()];
            for (int i = 0; i < list.size(); i++) {
                Object item = list.get(i);
                if (item instanceof Number num) {
                    bytes[i] = num.byteValue();
                } else {
                    throw new RuntimeException("toBytes() list must contain only numbers, got: " + item.getClass().getName() + " at index " + i);
                }
            }
            return bytes;
        };
    }

    static JavaInvokable toJson() {
        return args -> {
            if (args.length < 1) {
                throw new RuntimeException("toJson() needs at least one argument");
            }
            Object obj = args[0];
            boolean removeNulls = args.length > 1 && Boolean.TRUE.equals(args[1]);
            Object result = Json.of(obj).value();
            if (removeNulls) {
                removeNullValues(result);
            }
            return result;

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