diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 738b7ef..dde9196 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -102,6 +102,7 @@
 void input_event(int event, enum e1inp_sign_type type, struct gsm_bts_trx *trx)
 {}
 
+static void queue_for_msc(struct bsc_msc_connection *con, struct msgb *msg) __attribute__((nonnull (1, 2)));
 static void queue_for_msc(struct bsc_msc_connection *con, struct msgb *msg)
 {
 	if (write_queue_enqueue(&con->write_queue, msg) != 0) {