From d4b3364bba12f19c0826aff7cb24b466dc88486c Mon Sep 17 00:00:00 2001
From: Nuno Campos <nuno@langchain.dev>
Date: Mon, 20 May 2024 12:00:10 -0700
Subject: [PATCH] Raise error if cond edge returns None

---
 langgraph/graph/graph.py | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/langgraph/graph/graph.py b/langgraph/graph/graph.py
index 6595238c2..5659b5031 100644
--- a/langgraph/graph/graph.py
+++ b/langgraph/graph/graph.py
@@ -92,6 +92,8 @@ class Branch(NamedTuple):
             destinations = [self.ends[r] for r in result]
         else:
             destinations = result
+        if any(dest is None for dest in destinations):
+            raise ValueError("Branch did not return a valid destination")
         return writer(destinations) or input