diff --git a/toolkit/components/places/public/nsINavHistoryService.idl b/toolkit/components/places/public/nsINavHistoryService.idl
index 4c14da7f45ea..b345a087bca8 100644
--- a/toolkit/components/places/public/nsINavHistoryService.idl
+++ b/toolkit/components/places/public/nsINavHistoryService.idl
@@ -245,6 +245,8 @@ interface nsINavHistoryContainerResultNode : nsINavHistoryResultNode
    * This gives you the children of the nodes. It is preferrable to use this
    * interface over the array one, since it avoids creating an nsIArray object
    * and the interface is already the correct type.
+   *
+   * @throws NS_ERROR_NOT_AVAILABLE if containerOpen is false.
    */
   readonly attribute PRUint32 childCount;
   nsINavHistoryResultNode getChild(in PRUint32 index);