diff --git a/src/policy_cache.hpp b/src/policy_cache.hpp
index e65b9315..4307473d 100644
--- a/src/policy_cache.hpp
+++ b/src/policy_cache.hpp
@@ -25,6 +25,7 @@
 #include "nonstd/optional.hpp"
 #include "boost/unordered/concurrent_flat_map.hpp"
 #include "boost/flyweight.hpp"
+#include "boost/flyweight/no_tracking.hpp"
 #include "wyhash.h"
 #include "strpool.h"