diff --git a/g10/call-keyboxd.c b/g10/call-keyboxd.c
index 121b7aa2a..507c05ce6 100644
--- a/g10/call-keyboxd.c
+++ b/g10/call-keyboxd.c
@@ -223,7 +223,9 @@ open_context (ctrl_t ctrl, keyboxd_local_t *r_kbl)
           return err;
         }
 
-      err = kbx_client_data_new (&kbl->kcd, kbl->ctx, 1);
+      /* We use D-lines in 2.4 for communication due to a bug with fd
+       * passing.  See T6512.  */
+      err = kbx_client_data_new (&kbl->kcd, kbl->ctx, 1 /*=use D-lines*/);
       if (err)
         {
           assuan_release (kbl->ctx);
diff --git a/sm/keydb.c b/sm/keydb.c
index 411720513..b64e3f1df 100644
--- a/sm/keydb.c
+++ b/sm/keydb.c
@@ -580,6 +580,8 @@ open_context (ctrl_t ctrl, keydb_local_t *r_kbl)
           return err;
         }
 
+      /* We use D-lines in 2.4 for communication due to a bug with fd
+       * passing.  See T6512.  */
       err = kbx_client_data_new (&kbl->kcd, kbl->ctx, 1);
       if (err)
         {