diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 4b7ae1f..766223f 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -4160,7 +4160,8 @@
      *
      * The `psa_tls12_prf_key_derivation` structure saves the block
      * `HMAC_hash(secret, A(i) + seed)` from which the output
-     * is currently extracted as `output_block`.
+     * is currently extracted as `output_block` and where i is
+     * `block_number`.
      */
 
     /* Save the hash context before using it, to preserve the hash state with