diff --git a/lib/idr.c b/lib/idr.c
index fb86a67..5e0966b 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -589,7 +589,8 @@
  * @nextidp:  pointer to lookup key
  *
  * Returns pointer to registered object with id, which is next number to
- * given id.
+ * given id. After being looked up, *@nextidp will be updated for the next
+ * iteration.
  */
 
 void *idr_get_next(struct idr *idp, int *nextidp)