diff --git a/src/csspring/__init__.py b/src/csspring/__init__.py
index de39ab0..4911939 100644
--- a/src/csspring/__init__.py
+++ b/src/csspring/__init__.py
@@ -1,14 +1,14 @@
 """Parsing of CSS text aligned with [CSS] specification(s)."""
 
-def _enable_selector_parsing():
+def _enable_selector_parsing() -> None:
     """Augment parsing procedures to enable parsing of CSS selectors.
 
     This adds on-demand parsing of the prelude part of every qualified rule (see the `prelude` attribute on `QualifiedRule`), when the `selector_list` property is accessed on the latter. Because parsing is only done when the property added with this procedure, is accessed, `csspring.syntax` remains compliant with its respective specification.
     """
-    from .syntax.parsing import normalize_input, QualifiedRule
+    from .syntax.parsing import normalize_input, Product, QualifiedRule, tokens
     from .selectors import parse_selector_list
-    def qualified_rule_selector_list(self):
-        return parse_selector_list(normalize_input(self.prelude))
+    def qualified_rule_selector_list(self: QualifiedRule) -> Product | None:
+        return parse_selector_list(normalize_input(tokens(self.prelude)))
     setattr(QualifiedRule, "selector_list", property(qualified_rule_selector_list))
 
 _enable_selector_parsing()