diff --git a/glib/gspawn.c b/glib/gspawn.c
index a48b8d3a3..9cff4ed31 100644
--- a/glib/gspawn.c
+++ b/glib/gspawn.c
@@ -129,6 +129,7 @@ close_and_invalidate (gint *fd)
   return ret;
 }
 
+/* Some versions of OS X define READ_OK in public headers */
 #undef READ_OK
 
 typedef enum