--- a/toolkit/components/places/PlacesUtils.jsm
+++ b/toolkit/components/places/PlacesUtils.jsm
@@ -1073,16 +1073,19 @@ this.PlacesUtils = {
    *          True to make query items expand as new containers. For managing,
    *          you want this to be false, for menus and such, you want this to
    *          be true.
    * @returns A nsINavHistoryResult containing the contents of the
    *          folder. The result.root is guaranteed to be open.
    */
   getFolderContents:
   function PU_getFolderContents(aFolderId, aExcludeItems, aExpandQueries) {
+    if (typeof aFolderId !== "number") {
+      throw new Error("aFolderId should be a number.");
+    }
     var query = this.history.getNewQuery();
     query.setFolders([aFolderId], 1);
     var options = this.history.getNewQueryOptions();
     options.excludeItems = aExcludeItems;
     options.expandQueries = aExpandQueries;
 
     var result = this.history.executeQuery(query, options);
     result.root.containerOpen = true;