diff --git a/libi2pd/Garlic.h b/libi2pd/Garlic.h
index 718add61..e2b23a4b 100644
--- a/libi2pd/Garlic.h
+++ b/libi2pd/Garlic.h
@@ -19,6 +19,10 @@
 #include <mutex>
 #include <memory>
 #include <random>
+#include <boost/version.hpp>
+#if (BOOST_VERSION >= 108100) // 1.81
+#include <boost/unordered/unordered_flat_map.hpp>
+#endif
 #include "Crypto.h"
 #include "I2NPProtocol.h"
 #include "LeaseSet.h"
@@ -329,7 +333,11 @@ namespace garlic
 			// incoming
 			int m_NumRatchetInboundTags;
 			std::unordered_map<SessionTag, std::shared_ptr<AESDecryption>, std::hash<i2p::data::Tag<32> > > m_Tags;
+#if (BOOST_VERSION >= 108100) // 1.81
+			boost::unordered_flat_map<uint64_t, ECIESX25519AEADRatchetIndexTagset> m_ECIESx25519Tags; // session tag -> session
+#else
 			std::unordered_map<uint64_t, ECIESX25519AEADRatchetIndexTagset> m_ECIESx25519Tags; // session tag -> session
+#endif
 			// DeliveryStatus
 			std::mutex m_DeliveryStatusSessionsMutex;
 			std::unordered_map<uint32_t, GarlicRoutingSessionPtr> m_DeliveryStatusSessions; // msgID -> session