What causes "render() read arg should not be null" in karatelabs/karate?

Check the variable for null before calling render() and provide a default path

karatelabs/karate · error · RuntimeException

render() read arg should not be null

Error message

render() read arg should not be null

What it means

When karate.render() receives a single argument that is explicitly null (and not a Map or string), the library cannot determine what to render and throws this error. A null first argument is distinct from a missing one, so it gets its own guard.

Solutions

Check the variable for null before calling render() and provide a default path
Pass a string or { read: '...' } map instead of null
Trace where the null came from (missing config key, absent response field) and fix the source.

Example fix

// before
var path = karate.get('reportPath'); // may be null
var html = karate.render(path);
// after
var path = karate.get('reportPath') || 'report.feature';
var html = karate.render(path);

Defensive patterns

Strategy: type-guard

Validate before calling

// JS
if (path == null) path = 'report.feature';
var html = karate.render(path);

Type guard

function isNonNullTemplate(a) { return a != null; }

Try / catch

try { var html = karate.render(arg); } catch (e) { if (String(e).indexOf('should not be null') >= 0) karate.warn('null template path'); throw e; }

Prevention

Default-null template path variables before use
Don't pass config lookups straight into render() without a null check
Log/inspect the variable when a null path is possible

When it happens

Trigger: `karate.render(null)` directly, or `karate.render(someVar)` where someVar evaluates to null at runtime (e.g. a config value or response field that was null).

Common situations: A variable holding the template path was never set or came back null from a previous API call; reading a key from a config map that does not exist yields null and is then passed straight to render().

Related errors

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/eb2e8e8707a924d1. Report an issue: GitHub.

Appendix: source

Thrown at karate-core/src/main/java/io/karatelabs/core/KarateJs.java:1108

    }

    /**
     * karate.render(template) - Render HTML template (similar to doc).
     * Returns the rendered HTML string without sending to doc consumer.
     */
    @SuppressWarnings("unchecked")
    private JavaInvokable render() {
        return args -> {
            if (args.length == 0) {
                throw new RuntimeException("render() needs at least one argument");
            }
            String readPath;
            Map<String, Object> vars = null;
            if (args[0] instanceof Map) {
                Map<String, Object> map = (Map<String, Object>) args[0];
                readPath = (String) map.get("read");
            } else if (args[0] == null) {
                throw new RuntimeException("render() read arg should not be null");
            } else {
                readPath = args[0] + "";
            }
            if (args.length > 1 && args[1] instanceof Map) {
                vars = (Map<String, Object>) args[1];
            }
            return markup().processPath(readPath, vars);
        };
    }

    /**
     * karate.toAbsolutePath(path) - Convert a relative path to absolute.
     * Resolves relative to the current feature file's directory.
     */
    private JavaInvokable toAbsolutePath() {
        return args -> {
            if (args.length == 0) {
                throw new RuntimeException("toAbsolutePath() needs a path argument");

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