diff --git a/stsg/build.py b/stsg/build.py
index d15a03a..332a01f 100644
--- a/stsg/build.py
+++ b/stsg/build.py
@@ -100,7 +100,7 @@ class ArticleOverview:
             exit(1)
         ARTICLE_LAKE[self.slug] = self
 
-        self.url = url + "/" + self.slug
+        self.url: str = url + "/" + self.slug
         self.dist_path = Path(DIST_DIRECTORY, self.url.strip("/"))
 
         self.child_articles: List[ArticleOverview] = []