karatelabs/karate · error · RuntimeException

missing argument for header()

Error message

missing argument for header()

What it means

Generic arity guard inside HttpRequest's private header() invokable: it fires when header() is invoked with zero arguments. The 1-arg form reads a header value and the 2-arg form sets/removes one, so a no-arg call has neither a header name nor value to act on and this sentinel RuntimeException is thrown.

Solutions

1. Pass the header name to read: request.header('Authorization')
2. Pass name and value to set: request.header('X-Token', 'abc') (null value removes it, List sets all values)
3. Verify the variable holding the header name is non-empty

Example fix

// before
var h = request.header();
// after
var h = request.header('Content-Type');

if (name == null || name.isEmpty()) throw new IllegalArgumentException("header name required");

function requireArg(name, args) { if (args == null || args.length === 0) throw new Error('missing argument for header()'); return name; }

try { var h = request.header('X-Token'); } catch (RuntimeException e) { /* fix call site: header name missing */ }

When it happens

Trigger: Calling request.header() with no arguments, instead of request.header('X-Token') to read or request.header('X-Token', 'v') to set.

Common situations: Omitted header name while converting from setter-only usage, empty variable interpolated as the name, misunderstanding the overload dispatch.

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

• missing argument for headerValues()
• missing argument for param()
• missing argument for paramValues()
• missing argument for paramInt()
• missing argument for paramJson()

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/76f0e48cc82817ab. Report an issue: GitHub.

Appendix: source

Thrown at karate-core/src/main/java/io/karatelabs/http/HttpRequest.java:595

                    return Json.of(val).value();
                } catch (Exception e) {
                    return val; // return raw string if not valid JSON
                }
            } else {
                throw new RuntimeException("missing argument for paramJson()");
            }
        };
    }

    /**
     * 1-arg form: {@code request.header('X')} — returns the last value (String) or null.
     * 2-arg form: {@code request.header('X', 'v')} — sets the header. {@code null}
     * value removes the header. {@code List} value sets all values.
     */
    private JavaInvokable header() {
        return args -> {
            if (args.length == 0) {
                throw new RuntimeException("missing argument for header()");
            }
            String name = args[0] + "";
            if (args.length == 1) {
                return getHeader(name);
            }
            putHeaderValue(name, args[1]);
            return null;
        };
    }

    @SuppressWarnings("unchecked")
    void putHeaderValue(String name, Object value) {
        if (value == null) {
            removeHeader(name);
            return;
        }
        if (value instanceof List<?> list) {
            List<String> strList = new ArrayList<>(list.size());

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