diff --git a/src/evalwindow.c b/src/evalwindow.c
index bc59251..3578669 100644
--- a/src/evalwindow.c
+++ b/src/evalwindow.c
@@ -89,6 +89,7 @@
 
 /*
  * Return the window and tab pointer of window "id".
+ * Returns NULL when not found.
  */
     win_T *
 win_id2wp_tp(int id, tabpage_T **tpp)