diff --git a/src/api/sockets.c b/src/api/sockets.c
index a6f2dba7..0fb83a11 100644
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -230,7 +230,7 @@ struct lwip_socket_multicast_pair {
   ip4_addr_t multi_addr;
 };
 
-struct lwip_socket_multicast_pair socket_ipv4_multicast_memberships[LWIP_SOCKET_MAX_MEMBERSHIPS];
+static struct lwip_socket_multicast_pair socket_ipv4_multicast_memberships[LWIP_SOCKET_MAX_MEMBERSHIPS];
 
 static int  lwip_socket_register_membership(int s, const ip4_addr_t *if_addr, const ip4_addr_t *multi_addr);
 static void lwip_socket_unregister_membership(int s, const ip4_addr_t *if_addr, const ip4_addr_t *multi_addr);
@@ -249,7 +249,7 @@ struct lwip_socket_multicast_mld6_pair {
   ip6_addr_t multi_addr;
 };
 
-struct lwip_socket_multicast_mld6_pair socket_ipv6_multicast_memberships[LWIP_SOCKET_MAX_MEMBERSHIPS];
+static struct lwip_socket_multicast_mld6_pair socket_ipv6_multicast_memberships[LWIP_SOCKET_MAX_MEMBERSHIPS];
 
 static int  lwip_socket_register_mld6_membership(int s, unsigned int if_idx, const ip6_addr_t *multi_addr);
 static void lwip_socket_unregister_mld6_membership(int s, unsigned int if_idx, const ip6_addr_t *multi_addr);