diff --git a/include/polarssl/gcm.h b/include/polarssl/gcm.h
index 5a94722..3326919 100644
--- a/include/polarssl/gcm.h
+++ b/include/polarssl/gcm.h
@@ -186,8 +186,8 @@
  *                  16 bytes.
  *
  * \param ctx       GCM context
- * \param tag       buffer for holding the tag (may be NULL if tag_len is 0)
- * \param tag_len   length of the tag to generate
+ * \param tag       buffer for holding the tag
+ * \param tag_len   length of the tag to generate (must be at least 4)
  *
  * \return          0 if successful or POLARSSL_ERR_GCM_BAD_INPUT
  */