diff --git a/dulwich/repo.py b/dulwich/repo.py
index 7c906429e..db3a7c1a7 100644
--- a/dulwich/repo.py
+++ b/dulwich/repo.py
@@ -447,7 +447,9 @@ def get_graph_walker(self, heads=None):
         :return: A graph walker object
         """
         if heads is None:
-            heads = self.refs.as_dict(b'refs/heads').values()
+            heads = [
+                sha for sha in self.refs.as_dict(b'refs/heads').values()
+                if sha in self.object_store]
         return ObjectStoreGraphWalker(
             heads, self.get_parents, shallow=self.get_shallow())