diff --git a/common/cryptomsg.c b/common/cryptomsg.c
index 8c4bd93c2..1b50a1f18 100644
--- a/common/cryptomsg.c
+++ b/common/cryptomsg.c
@@ -118,7 +118,8 @@ u8 *cryptomsg_decrypt_body(const tal_t *ctx,
 	return decrypted;
 }
 
-bool cryptomsg_decrypt_header(struct crypto_state *cs, u8 hdr[18], u16 *lenp)
+bool cryptomsg_decrypt_header(struct crypto_state *cs, const u8 hdr[18],
+			      u16 *lenp)
 {
 	unsigned char npub[crypto_aead_chacha20poly1305_ietf_NPUBBYTES];
 	unsigned long long mlen;
diff --git a/common/cryptomsg.h b/common/cryptomsg.h
index 19c6d3e90..3da33879c 100644
--- a/common/cryptomsg.h
+++ b/common/cryptomsg.h
@@ -25,7 +25,8 @@
 u8 *cryptomsg_encrypt_msg(const tal_t *ctx,
 			  struct crypto_state *cs,
 			  const u8 *msg);
-bool cryptomsg_decrypt_header(struct crypto_state *cs, u8 hdr[18], u16 *lenp);
+bool cryptomsg_decrypt_header(struct crypto_state *cs, const u8 hdr[18],
+			      u16 *lenp);
 u8 *cryptomsg_decrypt_body(const tal_t *ctx,
 			   struct crypto_state *cs, const u8 *in);
 #endif /* LIGHTNING_COMMON_CRYPTOMSG_H */