diff --git a/src/backend/html_markdown.py b/src/backend/html_markdown.py
index a159ea6f..4f55b076 100644
--- a/src/backend/html_markdown.py
+++ b/src/backend/html_markdown.py
@@ -268,11 +268,11 @@ class HTMLProcessor:
         return {
             "tags": inline_tags if inline else all_tags,
             "attributes": inlines_attributes if inline else attributes,
-            "empty": {} if inline else {"hr", "br", "img"},
-            "separate": {"a"} if inline else {
+            "empty": set() if inline else set({"hr", "br", "img"}),
+            "separate": set({"a"}) if inline else set({
                 "a", "p", "li", "table", "tr", "th", "td", "br", "hr", "img",
-            },
-            "whitespace": {},
+            }),
+            "whitespace": set(),
             "keep_typographic_whitespace": True,
             "add_nofollow": False,
             "autolink": {