diff --git a/src/diff.c b/src/diff.c
index 257e18f..a3a3edd 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -189,7 +189,7 @@
 /**
  * @brief Get a userord entry for a specific user-ordered list/leaf-list. Create if does not exist yet.
  *
- * @param[in] first
+ * @param[in] first Node from the first tree, can be NULL (on create).
  * @param[in] schema Schema node of the list/leaf-list.
  * @param[in,out] userord Sized array of userord items.
  * @return Userord item for all the user-ordered list/leaf-list instances.