diff --git a/src/calibre/library/server/ajax.py b/src/calibre/library/server/ajax.py
index 29bbef5653..6a594961cd 100644
--- a/src/calibre/library/server/ajax.py
+++ b/src/calibre/library/server/ajax.py
@@ -372,7 +372,7 @@ def ajax_category(self, name, sort='title', num=100, offset=0,
             'count': x.count,
             'url': absurl(self.opts.url_prefix, '/ajax/books_in/%s/%s'%(
                 x.category if x.category else toplevel,
-                x.original_name)),
+                x.id if x.id is not None else x.original_name)),
             'has_children': x.original_name in children,
             } for x in items]