diff --git a/third-party/libutp/README.md b/third-party/libutp/README.md
index d7fca4c66..83270e69e 100644
--- a/third-party/libutp/README.md
+++ b/third-party/libutp/README.md
@@ -45,6 +45,13 @@ from libutp:
 
     cd utp_test && make
 
+## Packaging and API
+
+The libutp API is considered unstable, and probably always will be. We encourage
+you to test with the version of libutp you have, and be mindful when upgrading.
+For this reason, it is probably also a good idea to bundle libutp with your
+application.
+
 ## License
 
 libutp is released under the [MIT][lic] license.
diff --git a/third-party/libutp/utp.cpp b/third-party/libutp/utp.cpp
index 17d6e720d..db99c8ffa 100644
--- a/third-party/libutp/utp.cpp
+++ b/third-party/libutp/utp.cpp
@@ -120,7 +120,7 @@ struct PACKED_ATTRIBUTE PackedSockAddr {
 
 	byte get_family() const
 	{
-		return (IN6_IS_ADDR_V4MAPPED((in6_addr*)_sin6) != 0) ? AF_INET : AF_INET6;
+		return (IN6_IS_ADDR_V4MAPPED(&_in._in6addr) != 0) ? AF_INET : AF_INET6;
 	}
 
 	bool operator==(const PackedSockAddr& rhs) const