diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 96e374e..bfa9446 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -4433,7 +4433,7 @@
 /*
  * Return 0 if sequence number is acceptable, -1 otherwise
  */
-int mbedtls_ssl_dtls_replay_check( mbedtls_ssl_context *ssl )
+int mbedtls_ssl_dtls_replay_check( mbedtls_ssl_context const *ssl )
 {
     uint64_t rec_seqnum = ssl_load_six_bytes( ssl->in_ctr + 2 );
     uint64_t bit;