flowable/flowable-engine · error · FlowableException

Expression did not evaluate to a valid value (non empty…

Error message

Expression ${jobTopicExpression} did not evaluate to a valid value (non empty String). Was: ${topicValue}. For ${execution}

What it means

External worker task behavior guard: the expression configured as the job topic evaluated to null or an empty string for the current execution, so no external worker job can be created and the behavior fails.

Solutions

1. Fix the jobTopicExpression so it resolves to a non-empty string
2. Use a static topic value instead of an expression
3. Ensure variables referenced by the expression are set on the execution before the task

When it happens

Trigger: Thrown at modules/flowable-engine/src/main/java/org/flowable/engine/impl/bpmn/behavior/ExternalWorkerTaskActivityBehavior.java:123 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/4e0dcd7c98794f08. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-engine/src/main/java/org/flowable/engine/impl/bpmn/behavior/ExternalWorkerTaskActivityBehavior.java:123

            job.setRetries(jobServiceConfiguration.getAsyncExecutorNumberOfRetries());

            // Inherit tenant id (if applicable)
            if (execution.getTenantId() != null) {
                job.setTenantId(execution.getTenantId());
            }

            Expression jobTopicExpression;
            if (StringUtils.isEmpty(beforeContext.getJobTopicExpression())) {
                jobTopicExpression = this.jobTopicExpression;
            } else {
                jobTopicExpression = processEngineConfiguration.getExpressionManager()
                        .createExpression(beforeContext.getJobTopicExpression());
            }
            Object topicValue = jobTopicExpression.getValue(execution);
            if (topicValue != null && !topicValue.toString().isEmpty()) {
                job.setJobHandlerConfiguration(topicValue.toString());
            } else {
                throw new FlowableException("Expression " + jobTopicExpression + " did not evaluate to a valid value (non empty String). Was: " + topicValue + ". For " + execution);
            }

            jobService.insertExternalWorkerJob(job);

            if (interceptor != null) {
                interceptor.afterCreateExternalWorkerJob(new CreateExternalWorkerJobAfterContext(
                        (ExternalWorkerServiceTask) currentFlowElement,
                        job,
                        execution
                ));
            }
        } else {
            leave(execution);
        }
    }

    @Override
    public void trigger(DelegateExecution execution, String signalName, Object signalData) {

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