diff --git a/src/gsm/cbsp.c b/src/gsm/cbsp.c
index c2a1b8d..81ba81e 100644
--- a/src/gsm/cbsp.c
+++ b/src/gsm/cbsp.c
@@ -1261,6 +1261,7 @@
  *  \returns callee-allocated decoded representation of CBSP message; NULL on error */
 struct osmo_cbsp_decoded *osmo_cbsp_decode(void *ctx, struct msgb *in)
 {
+	OSMO_ASSERT(in->l1h != NULL && in->l2h != NULL);
 	struct osmo_cbsp_decoded *out = talloc_zero(ctx, struct osmo_cbsp_decoded);
 	const struct cbsp_header *h = msgb_l1(in);
 	struct tlv_parsed tp[16]; /* max. number of pages in a given CBS message */