src/Pure/System/options.scala

--- a/src/Pure/System/options.scala	Mon Jun 07 09:36:21 2021 +0200
+++ b/src/Pure/System/options.scala	Mon Jun 07 11:42:05 2021 +0200
@@ -349,7 +349,7 @@
     val opt = check_name(name)
     opt_value match {
       case Some(value) => this + (name, value)
-      case None if opt.typ == Options.Bool => this + (name, "true")
+      case None if opt.typ == Options.Bool | opt.typ == Options.String => this + (name, "true")
       case None => error("Missing value for option " + quote(name) + " : " + opt.typ.print)
     }
   }