diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index a65143c..3b4bfad 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1705,6 +1705,9 @@
 
 /*
  * Compute HMAC of variable-length data with constant flow.
+ *
+ * Only works with MD-5, SHA-1, SHA-256 and SHA-384.
+ * (Otherwise, computation of block_size needs to be adapted.)
  */
 int mbedtls_ssl_cf_hmac(
         mbedtls_md_context_t *ctx,