Java checkstyle error ParameterAssignmentCheck

Question

private int getChildChildDepth(List<ProjectTask> childProjectTasks, int count)
    {
        for (ProjectTask projectTask : childProjectTasks)
        {
            if (projectTask.getChildTasks().size() > 0)
            {
                count++;
                getChildChildDepth(projectTask.getChildTasks(), count);
            }
        }
        return count;
    }

In above i am getting ParameterAssignmentCheck error at count++. I am using recursive function. Error: Assignment of parameter 'count' is not allowed.