diff --git a/frontend/auth_providers/sustech.py b/frontend/auth_providers/sustech.py
index e6e467d..ea20eb4 100644
--- a/frontend/auth_providers/sustech.py
+++ b/frontend/auth_providers/sustech.py
@@ -28,7 +28,7 @@ def login_attrs(self) -> dict[str, Any]:
 
     def check_ticket(self) -> Optional[ElementTree.Element]:
         tree = super().check_ticket()
-        if not tree:
+        if tree is None:
             return None
         self.identity = tree.find(self.YALE_CAS_URL + 'user').text.strip()
         self.email = tree.find(self.YALE_CAS_URL + 'attributes').find(self.YALE_CAS_URL + 'mail').text.strip()
diff --git a/frontend/auth_providers/ustc.py b/frontend/auth_providers/ustc.py
index 5dd9779..2cd79df 100644
--- a/frontend/auth_providers/ustc.py
+++ b/frontend/auth_providers/ustc.py
@@ -26,12 +26,13 @@ def login_attrs(self) -> dict[str, Any]:
 
     def check_ticket(self) -> Optional[ElementTree.Element]:
         tree = super().check_ticket()
-        if not tree:
+        if tree is None:
             return None
-        self.identity = tree.find(self.YALE_CAS_URL + 'attributes').find(self.YALE_CAS_URL + 'gid').text.strip()
-        if not self.identity:
+        attributes = tree.find(self.YALE_CAS_URL + 'attributes')
+        if attributes is None:
             # compatibility with old ustc cas
-            self.identity = tree.find('attributes').find(self.YALE_CAS_URL + 'gid').text.strip()
+            attributes = tree.find('attributes')
+        self.identity = attributes.find(self.YALE_CAS_URL + 'gid').text.strip()
         self.sno = tree.find(self.YALE_CAS_URL + 'user').text.strip()
         return tree