diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index 2ddefc3..7d0559b 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -446,6 +446,7 @@
 
     mbedtls_x509_crt_init( ssl->session_negotiate->peer_cert );
 
+    MBEDTLS_SSL_CHK_BUF_READ_PTR( p, end, certificate_list_len );
     certificate_list_end = p + certificate_list_len;
     while( p < certificate_list_end )
     {