flowable/flowable-engine · error · FlowableException

condition script returns null: for

Error message

condition script returns null:  for 

What it means

FlowableException thrown by ScriptCondition.evaluate when a condition expression script executes but its result is null. Conditional sequence flows (e.g. exclusive gateways) require a Boolean answer, so a null result makes the routing decision impossible.

Solutions

1. Make the condition script explicitly return a boolean (e.g. return amount > 100 in groovy/js).
2. Check the referenced process variables exist and are non-null before the gateway, or use null-safe defaults in the expression.
3. Verify the condition language is correct for the expression semantics; inspect the execution object in the message to see which variables were present.
4. Catch FlowableException around the engine call / API invocation and log the expression for diagnosis.

Example fix

// before (groovy condition, may return null)
amount

// after
return amount != null ? amount > 100 : false

Object amount = execution.getVariable("amount");
if (amount == null) {
    execution.setVariable("amount", BigDecimal.ZERO);
}

try {
    boolean take = condition.evaluate(scriptingEngines, execution, language, expression);
} catch (FlowableException e) {
    log.error("condition '{}' failed for {}: {}", expression, execution, e.getMessage());
    throw e;
}

When it happens

Trigger: A gateway/boundary condition expression like ${language} (or a script condition with a language such as groovy/javascript) whose script evaluates to null — e.g. returning a missing variable, an empty return, or a method returning null.

Common situations: Referencing a process variable that is not set; script body with no explicit return statement; a helper method returning null for edge-case inputs; typo in variable name so EL resolves to null.

Understand the failure class

Background: "invalid response format", "malformed payload", "missing data field": when an API returns 200 but the response shape is wrong — this error's family across 23 libraries.

Related errors

• condition script returns non-Boolean: () for
• Class or class name is missing
• condition expression returns non-Boolean
• condition expression returns null
• condition script returns non-Boolean

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/2f30f340db31f224. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-engine/src/main/java/org/flowable/engine/impl/scripting/ScriptCondition.java:45

    private final String expression;
    private final String language;

    public ScriptCondition(String expression, String language) {
        this.expression = expression;
        this.language = language;
    }

    @Override
    public boolean evaluate(String elementId, DelegateExecution execution) {
        ScriptingEngines scriptingEngines = CommandContextUtil.getProcessEngineConfiguration().getScriptingEngines();

        ScriptEngineRequest.Builder builder = ScriptEngineRequest.builder()
                .script(expression)
                .language(language)
                .scopeContainer(execution);
        Object result = scriptingEngines.evaluate(builder.build()).getResult();
        if (result == null) {
            throw new FlowableException("condition script returns null: " + expression + " for " + execution);
        }
        if (!(result instanceof Boolean)) {
            throw new FlowableException("condition script returns non-Boolean: " + result + " (" + result.getClass().getName() + ") for " + execution);
        }
        return (Boolean) result;
    }

}

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