diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index f68643d..6a49c39 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -143,7 +143,7 @@
  * \param iv_len    The length of the IV.
  * \param add       The buffer holding the additional data. If \p add_len is
  *                  greater than zero, this must be of at least that size in
- *                  Bytes.
+ *                  Bytes. If `add_len == 0`, this may be \c NULL.
  * \param add_len   The length of the additional data.
  * \param input     The buffer holding the input data. If \p length is greater
  *                  than zero, this must be a readable buffer of at least that
@@ -190,7 +190,7 @@
  * \param iv_len    The length of the IV.
  * \param add       The buffer holding the additional data. If \p add_len is
  *                  greater than zero, this must be of at least that size in
- *                  Bytes.
+ *                  Bytes. If `add_len == 0`, this may be \c NULL.
  * \param add_len   The length of the additional data.
  * \param tag       The buffer holding the tag to verify. This must be a
  *                  readable buffer of at least \p tag_len Bytes.
@@ -229,10 +229,10 @@
  * \param iv        The initialization vector. This must be a readable buffer of
  *                  at least \p iv_len Bytes.
  * \param iv_len    The length of the IV.
- * \param add       The buffer holding the additional data, or NULL
- *                  if \p add_len is 0.
- * \param add_len   The length of the additional data. If 0,
- *                  \p add may be NULL.
+ * \param add       The buffer holding the additional data, or \c NULL
+ *                  if \p add_len is \c 0.
+ * \param add_len   The length of the additional data. If \c 0,
+ *                  \p add may be \c NULL.
  *
  * \return          \c 0 on success.
  */