diff --git a/library/ssl_tls13_keys.c b/library/ssl_tls13_keys.c
index 9e629cb..7aec21d 100644
--- a/library/ssl_tls13_keys.c
+++ b/library/ssl_tls13_keys.c
@@ -113,17 +113,17 @@
 #endif
 
     *p++ = 0;
-    *p++ = MBEDTLS_CHAR_0( desired_length );
+    *p++ = MBEDTLS_BYTE_0( desired_length );
 
     /* Add label incl. prefix */
-    *p++ = MBEDTLS_CHAR_0( total_label_len );
+    *p++ = MBEDTLS_BYTE_0( total_label_len );
     memcpy( p, tls1_3_label_prefix, sizeof(tls1_3_label_prefix) );
     p += sizeof(tls1_3_label_prefix);
     memcpy( p, label, llen );
     p += llen;
 
     /* Add context value */
-    *p++ = MBEDTLS_CHAR_0( clen );
+    *p++ = MBEDTLS_BYTE_0( clen );
     if( clen != 0 )
         memcpy( p, ctx, clen );