diff --git a/src/tree_data.h b/src/tree_data.h
index dddeb6c..2c3066d 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -2144,7 +2144,7 @@
  * @param[in] first First data tree.
  * @param[in] second Second data tree.
  * @param[in] options Bitmask of options flags, see @ref diffoptions.
- * @param[out] diff Generated diff.
+ * @param[out] diff Generated diff, NULL if there are no differences.
  * @return LY_SUCCESS on success,
  * @return LY_ERR on error.
  */
@@ -2159,7 +2159,7 @@
  * @param[in] first First data tree.
  * @param[in] second Second data tree.
  * @param[in] options Bitmask of options flags, see @ref diffoptions.
- * @param[out] diff Generated diff.
+ * @param[out] diff Generated diff, NULL if there are no differences.
  * @return LY_SUCCESS on success,
  * @return LY_ERR on error.
  */