diff --git a/include/mbedtls/cipher.h b/include/mbedtls/cipher.h
index 4a71383..2d609db 100644
--- a/include/mbedtls/cipher.h
+++ b/include/mbedtls/cipher.h
@@ -682,7 +682,7 @@
  * \param ctx           The generic cipher context. This must be initialized.
  * \param ad            The additional data to use. This must be a readable
  *                      buffer of at least \p ad_len Bytes.
- * \param ad_len        The Length of \p ad Bytes.
+ * \param ad_len        The length of \p ad in Bytes.
  *
  * \return              \c 0 on success.
  * \return              A specific error code on failure.