diff -r 39c90514faf8 -r a56eab490f4e src/Pure/PIDE/headless.scala
--- a/src/Pure/PIDE/headless.scala	Sat Aug 31 21:34:39 2019 +0200
+++ b/src/Pure/PIDE/headless.scala	Sun Sep 01 22:57:25 2019 +0200
@@ -492,8 +492,8 @@
       deps: Sessions.Deps, progress: Progress = No_Progress): List[Document.Node.Name] =
     {
       for {
-        (_, name) <- deps.used_theories_condition(options, progress = progress)
-        if !session_base.loaded_theory(name)
+        name <- deps.used_theories_condition(options, progress = progress).topological_order
+        if !session_base.loaded_theory(name.theory)
       } yield name
     }