diff --git a/src/net/IPv4Address.hxx b/src/net/IPv4Address.hxx
index 7e6ee79f4..e57bdf4cf 100644
--- a/src/net/IPv4Address.hxx
+++ b/src/net/IPv4Address.hxx
@@ -172,7 +172,7 @@ public:
 	 * Return a downcasted reference to the address.  This call is
 	 * only legal after verifying SocketAddress::GetFamily().
 	 */
-	static constexpr const IPv4Address &Cast(const SocketAddress src) noexcept {
+	static constexpr const IPv4Address &Cast(const SocketAddress &src) noexcept {
 		/* this reinterpret_cast works because this class is
 		   just a wrapper for struct sockaddr_in6 */
 		return *(const IPv4Address *)(const void *)src.GetAddress();