karatelabs/karate · error · RuntimeException

toBytes() needs one argument: a list of numbers

Error message

toBytes() needs one argument: a list of numbers

What it means

karate.toBytes() converts its argument into a byte[] — numbers become bytes, an existing byte[] is returned as-is. The library throws this error when called with no argument at all, since there is nothing to convert. It is the arity guard before any type inspection happens.

Solutions

1. Pass the list of numbers: karate.toBytes([104, 101, 108, 108, 111]).
2. Check the variable holding the numbers list is still in scope and passed through.
3. If you already have a byte[] in Java, you can pass it directly — it is returned unchanged.

Example fix

// before
var bytes = karate.toBytes()
// after
var bytes = karate.toBytes([72, 73])

if (nums == null || !Array.isArray(nums)) { throw new Error('toBytes() requires a list of numbers, got: ' + nums) }

function isByteList(v) { return v instanceof Array && v.every(n => typeof n === 'number') }

try { var bytes = karate.toBytes(nums) } catch (e) { if (('' + e).indexOf('needs one argument') >= 0) { throw new Error('toBytes(): pass the numbers list explicitly') } throw e }

When it happens

Trigger: karate.toBytes() with zero arguments, e.g. a dropped argument after refactoring or calling the function reference without invoking arguments.

Common situations: Building binary request bodies (files, certificates) in tests; generating byte payloads dynamically where the source variable was removed or renamed.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• toBytes() argument must be a list of numbers, got
• toBytes() list must contain only numbers, got
• read() needs at least one argument
• sysenv() needs the environment-variable name
• sysprop() needs the property name

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/27c6f6bc7e8d6a19. Report an issue: GitHub.

Appendix: source

Thrown at karate-core/src/main/java/io/karatelabs/core/KarateJsUtils.java:559

            if (args.length == 0 || args[0] == null) {
                return "";
            }
            if (!(args[0] instanceof List)) {
                throw new RuntimeException("toCsv() argument must be a list of maps, got: " + args[0].getClass().getName());
            }
            List<Map<String, Object>> list = (List<Map<String, Object>>) args[0];
            if (list.isEmpty()) {
                return "";
            }
            return DataUtils.toCsv(list);
        };
    }

    @SuppressWarnings("unchecked")
    static JavaInvokable toBytes() {
        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);
                }
            }

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