diff -r 84f716e72fa3 -r d115d50a19c0 src/Pure/ML/ml_process.scala
--- a/src/Pure/ML/ml_process.scala	Fri Aug 07 22:28:04 2020 +0200
+++ b/src/Pure/ML/ml_process.scala	Fri Aug 07 22:57:14 2020 +0200
@@ -110,7 +110,9 @@
 
     // ISABELLE_TMP
     val isabelle_tmp = Isabelle_System.tmp_dir("process")
-    val env_tmp = Map("ISABELLE_TMP" -> File.standard_path(isabelle_tmp))
+    val env_tmp =
+      Map("ISABELLE_TMP" -> File.standard_path(isabelle_tmp),
+        "POLYSTATSDIR" -> isabelle_tmp.getAbsolutePath)
 
     val env_functions = Map("ISABELLE_SCALA_FUNCTIONS" -> Scala.functions.mkString(","))