diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 5352c7e68c42..1d7fad8269e6 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -164,6 +164,12 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 		rtnl_lock();
 	lock_sock(sk);
 
+	/* Another thread has converted the socket into IPv4 with
+	 * IPV6_ADDRFORM concurrently.
+	 */
+	if (unlikely(sk->sk_family != AF_INET6))
+		goto unlock;
+
 	switch (optname) {
 
 	case IPV6_ADDRFORM:
@@ -924,6 +930,7 @@ pref_skip_coa:
 		break;
 	}
 
+unlock:
 	release_sock(sk);
 	if (needs_rtnl)
 		rtnl_unlock();