diff --git a/seminar/treelib.py b/seminar/treelib.py
index 709e6d98..8ad33c9c 100644
--- a/seminar/treelib.py
+++ b/seminar/treelib.py
@@ -19,6 +19,8 @@ def safe_pred(node):
 		return None
 
 def first_brother(node):
+	if node is None:
+		return None
 	brother = node
 	while safe_pred(brother) is not None:
 		brother = safe_pred(brother)