diff --git a/astrbot/core/tools/message_tools.py b/astrbot/core/tools/message_tools.py
index 8106fed2c..4763907e1 100644
--- a/astrbot/core/tools/message_tools.py
+++ b/astrbot/core/tools/message_tools.py
@@ -27,12 +27,15 @@ from astrbot.core.utils.astrbot_path import (
 )
 
 
-def _file_send_allowed_roots(umo: str) -> tuple[Path, ...]:
-    return (
-        workspace_root(umo),
+def _file_send_allowed_roots(umo: str | None) -> tuple[Path, ...]:
+    roots = []
+    if umo:
+        roots.append(workspace_root(umo))
+    roots.extend([
         Path(get_astrbot_temp_path()).resolve(strict=False),
         Path(get_astrbot_system_tmp_path()).resolve(strict=False),
-    )
+    ])
+    return tuple(roots)
 
 
 def _is_path_within(path: Path, roots: tuple[Path, ...]) -> bool: