diff --git a/src/st/st-bin.c b/src/st/st-bin.c
index 4c862b62a..48347b6ba 100644
--- a/src/st/st-bin.c
+++ b/src/st/st-bin.c
@@ -545,7 +545,7 @@ st_bin_new (void)
 /**
  * st_bin_set_child:
  * @bin: a #StBin
- * @child: a #ClutterActor, or %NULL
+ * @child: (allow-none): a #ClutterActor, or %NULL
  *
  * Sets @child as the child of @bin.
  *