diff --git a/src/api/sockets.c b/src/api/sockets.c
index a37b21c7..0c11251a 100644
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -3693,12 +3693,16 @@ lwip_socket_drop_registered_mld6_memberships(int s)
   for (i = 0; i < LWIP_SOCKET_MAX_MEMBERSHIPS; i++) {
     if (socket_ipv6_multicast_memberships[i].sock == sock) {
       ip_addr_t multi_addr;
+      u8_t if_idx;
 
       ip_addr_copy_from_ip6(multi_addr, socket_ipv6_multicast_memberships[i].multi_addr);
+      if_idx = socket_ipv6_multicast_memberships[i].if_idx;
+
       socket_ipv6_multicast_memberships[i].sock   = NULL;
       socket_ipv6_multicast_memberships[i].if_idx = NETIF_NO_INDEX;
       ip6_addr_set_zero(&socket_ipv6_multicast_memberships[i].multi_addr);
-      netconn_join_leave_group_netif(sock->conn, &multi_addr, socket_ipv6_multicast_memberships[i].if_idx, NETCONN_LEAVE);
+
+      netconn_join_leave_group_netif(sock->conn, &multi_addr, if_idx, NETCONN_LEAVE);
     }
   }
   done_socket(sock);