diff --git a/2021/puzzle-16-01.cc b/2021/puzzle-16-01.cc
index cc3f592..72ac0cd 100644
--- a/2021/puzzle-16-01.cc
+++ b/2021/puzzle-16-01.cc
@@ -1,3 +1,4 @@
+#include <cassert>
 #include <iostream>
 #include <string>
 #include <vector>