flowable/flowable-engine · error · FlowableIllegalArgumentException

None of the given task assignees can be null

Error message

None of the given task assignees can be null

What it means

Flowable throws this when taskTaskAssigneeIds(...) (assignee-id list query) is called with a list containing at least one null element. The library validates every entry eagerly so a malformed IN-clause is never sent to the database. It is an immediate argument-validation failure, not a state or runtime issue.

Solutions

1. Filter null (and blank) entries before calling taskTaskAssigneeIds
2. Ensure upstream data does not produce null assignee ids (fix the query/source)
3. If you truly have no assignees, skip setting the filter instead of passing a list of nulls

Example fix

// before
query.taskTaskAssigneeIds(assigneeIds); // list may contain nulls
// after
List<String> clean = assigneeIds.stream().filter(Objects::nonNull).collect(Collectors.toList());
if (!clean.isEmpty()) { query.taskTaskAssigneeIds(clean); }

List<String> safeAssignees = assigneeIds == null ? Collections.emptyList() : assigneeIds.stream().filter(Objects::nonNull).collect(Collectors.toList());
if (safeAssignees.isEmpty()) throw new IllegalArgumentException("assigneeIds must contain at least one non-null id");

boolean isValidAssigneeIds(Collection<String> ids) {
    return ids != null && !ids.isEmpty() && ids.stream().allMatch(Objects::nonNull);
}

try {
    query.taskTaskAssigneeIds(assigneeIds);
} catch (FlowableIllegalArgumentException e) {
    // fall back to no assignee filter or corrected list
}

When it happens

Trigger: Calling HistoricTaskInstanceQuery.taskTaskAssigneeIds(Collection<String>) with a collection that contains a null element (e.g. a List built from a sparse map or split string with trailing separators).

Common situations: Collecting assignee ids from a database/HTTP join where some rows have NULL assignee; building the list from user input without filtering blanks; deserializing JSON arrays containing nulls.

Related errors

• Booleans and null cannot be used in 'greater than or equal'…
• Booleans and null cannot be used in 'less than or equal'…
• None of the given task categories can be null
• Task category list is null
• Task formKey is null

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/c6740dcf6ee48057. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-task-service/src/main/java/org/flowable/task/service/impl/HistoricTaskInstanceQueryImpl.java:947

            this.currentOrQueryObject.withAssignee = true;
        }
        else {
            this.withAssignee = true;
        }
        return this;
    }

    @Override
    public HistoricTaskInstanceQuery taskAssigneeIds(Collection<String> assigneeIds) {
        if (assigneeIds == null) {
            throw new FlowableIllegalArgumentException("Task assignee list is null");
        }
        if (assigneeIds.isEmpty()) {
            throw new FlowableIllegalArgumentException("Task assignee list is empty");
        }
        for (String assignee : assigneeIds) {
            if (assignee == null) {
                throw new FlowableIllegalArgumentException("None of the given task assignees can be null");
            }
        }

        if (taskAssignee != null) {
            throw new FlowableIllegalArgumentException("Invalid query usage: cannot set both taskAssigneeIds and taskAssignee");
        }
        if (taskAssigneeLike != null) {
            throw new FlowableIllegalArgumentException("Invalid query usage: cannot set both taskAssigneeIds and taskAssigneeLike");
        }
        if (taskAssigneeLikeIgnoreCase != null) {
            throw new FlowableIllegalArgumentException("Invalid query usage: cannot set both taskAssigneeIds and taskAssigneeLikeIgnoreCase");
        }

        if (inOrStatement) {
            currentOrQueryObject.taskAssigneeIds = assigneeIds;
        } else {
            this.taskAssigneeIds = assigneeIds;
        }

View on GitHub (pinned to d6d39ce1c6)