diff -r 7829d6435c60 -r c2ee8d993d6a src/Pure/Isar/obtain.ML
--- a/src/Pure/Isar/obtain.ML	Thu Sep 09 23:07:02 2021 +0200
+++ b/src/Pure/Isar/obtain.ML	Fri Sep 10 14:59:19 2021 +0200
@@ -335,7 +335,7 @@
           | _ => instT))));
     val closed_rule = rule
       |> Thm.forall_intr (Thm.cterm_of ctxt (Free thesis_var))
-      |> Thm.instantiate (TVars.dest instT, []);
+      |> Thm.instantiate (instT, Vars.empty);
 
     val ((_, rule' :: terms'), ctxt') = Variable.import false (closed_rule :: terms) ctxt;
     val vars' =