diff --git a/Sources/Avatar.php b/Sources/Avatar.php
index af5576de43..da1fdc3937 100644
--- a/Sources/Avatar.php
+++ b/Sources/Avatar.php
@@ -535,8 +535,14 @@ public function __construct(
 						// Resolving the canonical path of both the file and the
 						// directories is important in order to ensure that we
 						// don't end up navigating outside the avatar dirs.
+						// A forum at the root of its domain has no path of its
+						// own to strip. Url::$path is typed with no default, so
+						// it is not there to be read at all in that case, and
+						// reading it throws rather than giving back ''.
+						$board_path = Url::create(Config::$boardurl)->path ?? '';
+
 						$abs_path = Sapi::canonicalPath(
-							preg_replace('~^' . preg_quote(Url::create(Config::$boardurl)->path, '~') . '~u', '', $url->path),
+							preg_replace('~^' . preg_quote($board_path, '~') . '~u', '', $url->path),
 							base_dir: Config::$boarddir,
 						);