diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index 0a07e06..f5a6e20 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -116,6 +116,12 @@
 {
     int ret;
 
+    if( ssl->conf->f_rng == NULL )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "no RNG provided" ) );
+        return( MBEDTLS_ERR_SSL_NO_RNG );
+    }
+
     if( ( ret = ssl->conf->f_rng( ssl->conf->p_rng,
                                   ssl->handshake->randbytes,
                                   CLIENT_HELLO_RAND_BYTES_LEN ) ) != 0 )