Comments field not updating

Question

I am developing a workflow. I have extended the Workflow Task content type with the Comments field using SharePoint UI. I need to set some value for this field in my code when editing the task, but it stays empty. Here's my code:

private void SetTaskProperties(string comment)
{
    Hashtable taskProperties;

    taskProperties = new Hashtable();
    taskProperties.Add(SPBuiltInFieldId.Completed, true);
    taskProperties.Add(SPBuiltInFieldId.TaskStatus, completed);
    taskProperties.Add(SPBuiltInFieldId.PercentComplete, 1.0);
    taskProperties.Add(SPBuiltInFieldId.Comments, comment);
    SPWorkflowTask.AlterTask(this.task, taskProperties, false);
}

Should I be using something else than SPBuiltInFieldId.Comments? I am getting the same result when updating the task directly, instead of using the Hashtable in the code above.