From ef723cda9c4abf9c461804fec21dc649da209922 Mon Sep 17 00:00:00 2001
From: Samarium150 <28302241+Samarium150@users.noreply.github.com>
Date: Wed, 3 Dec 2025 13:25:27 -0700
Subject: [PATCH] fix: check vector size before calling back()

---
 environments/Witness.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/environments/Witness.h b/environments/Witness.h
index 50dc116cc..d9d20a553 100644
--- a/environments/Witness.h
+++ b/environments/Witness.h
@@ -2167,7 +2167,7 @@ void Witness<width, height>::ApplyAction(std::pair<int, int> &s, WitnessAction a
 template<int width, int height>
 void Witness<width, height>::ApplyAction(WitnessState<width, height> &s, WitnessAction a) const
 {
-    const auto &prevAction = s.leftRightRegions.back().action;
+    const auto prevAction = s.leftRightRegions.empty() ? kStart : s.leftRightRegions.back().action;
     auto leftRightRegion = LeftRightRegion{a};
     switch (a)
     {