diff --git a/gtk/gtktreemodel.c b/gtk/gtktreemodel.c
index 377a365d7b..0eef6a6474 100644
--- a/gtk/gtktreemodel.c
+++ b/gtk/gtktreemodel.c
@@ -731,9 +731,10 @@ gtk_tree_path_new_from_indicesv (int *indices,
  *
  * This string is a “:” separated list of numbers.
  * For example, “4:10:0:3” would be an acceptable
- * return value for this string.
+ * return value for this string. If the path has
+ * depth 0, %NULL is returned.
  *
- * Returns: A newly-allocated string.
+ * Returns: (nullable): A newly-allocated string.
  *     Must be freed with g_free().
  */
 char *
@@ -1316,7 +1317,7 @@ gtk_tree_model_get_iter_from_string (GtkTreeModel *tree_model,
  * For example, “4:10:0:3” would be an acceptable
  * return value for this string.
  *
- * Returns: a newly-allocated string.
+ * Returns: (nullable): a newly-allocated string.
  *     Must be freed with g_free().
  */
 char *