kestra-io/kestra · error · PebbleException

The 'loopOutputs' function expects 'name' to be a string.

Error message

The 'loopOutputs' function expects 'name' to be a string.

What it means

The loopOutputs() function checks that the 'name' argument is an instance of String. If name is non-null but is a number, boolean, or object, this type-check error fires. The name is used as a Map key to look up the output field in each iteration's outputs map, so it must be a string.

Source

Thrown at core/src/main/java/io/kestra/core/runners/pebble/functions/LoopOutputsFunction.java:50

        return ARGUMENT_DEFAULTS;
    }

    @Override
    public Object execute(Map<String, Object> args, PebbleTemplate self, EvaluationContext context, int lineNumber) {
        Object outputsArg = args.get(ARG_OUTPUTS);
        if (outputsArg == null) {
            throw new PebbleException(null, "The 'loopOutputs' function expects a non-null argument 'outputs'.", lineNumber, self.getName());
        }
        if (!(outputsArg instanceof List<?>)) {
            throw new PebbleException(null, "The 'loopOutputs' function expects 'outputs' to be a list.", lineNumber, self.getName());
        }

        Object nameArg = args.get(ARG_NAME);
        if (nameArg == null) {
            throw new PebbleException(null, "The 'loopOutputs' function expects a non-null argument 'name'.", lineNumber, self.getName());
        }
        if (!(nameArg instanceof String)) {
            throw new PebbleException(null, "The 'loopOutputs' function expects 'name' to be a string.", lineNumber, self.getName());
        }

        String key = (String) nameArg;
        List<?> loopOutputs = (List<?>) outputsArg;
        List<Object> result = new ArrayList<>(loopOutputs.size());

        for (Object loopOutputObj : loopOutputs) {
            if (loopOutputObj instanceof Map<?, ?> loopOutput) {
                Object iterationOutputs = loopOutput.get(ITERATION_OUTPUTS_KEY);
                result.add(iterationOutputs instanceof Map<?, ?> outputs ? outputs.get(key) : null);
            } else {
                result.add(null);
            }
        }

        return result;
    }
}

View on GitHub (pinned to 823fada927)

Solutions

1. Pass name as a quoted string literal or a string-typed variable: {{ loopOutputs(outputs.myLoop.outputs, 'result') }}.
2. If the name comes from a variable, ensure it is a string (quote it or convert it).

Example fix

# before
value: "{{ loopOutputs(outputs.myLoop.outputs, 42) }}"
# after
value: "{{ loopOutputs(outputs.myLoop.outputs, '42') }}"

# Ensure name is a string before calling:
{% set nameVal = myVar | string %}
{{ loopOutputs(outputs.myLoop.outputs, nameVal) }}

# Pebble type narrowing — check the value is a string:
{% if myVar is string %}...{% endif %}

When it happens

Trigger: Passing a numeric value as name (e.g. loopOutputs(outputs.loop, 42)). Passing a boolean or a complex object instead of a string literal or string variable.

Common situations: The author passes an unquoted value in YAML that Pebble evaluates as a Long instead of a String. A dynamic variable that holds a number is passed where a string key is expected.

Related errors

• The 'loopOutputs' function expects 'outputs' to be a list.
• The 'loopOutputs' function expects a non-null argument 'outp
• The 'loopOutputs' function expects a non-null argument 'name
• The 'randomIn()' function expects an argument %s of type Lon
• Unknown value element type: {}

AI-assisted analysis of kestra-io/kestra@823fada927 (2026-08-14). Data as JSON: /api/errors/b3296e58bc173f6d. Report an issue: GitHub.