diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 6739098..e647a9a 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -667,7 +667,7 @@
 
 #if defined(POLARSSL_SSL_ALPN)
 static int ssl_parse_alpn_ext( ssl_context *ssl,
-                               unsigned char *buf, size_t len )
+                               const unsigned char *buf, size_t len )
 {
     size_t list_len, cur_len, ours_len;
     const unsigned char *theirs, *start, *end;