src/Pure/Admin/build_cygwin.scala

--- a/src/Pure/Admin/build_cygwin.scala	Sat Jan 09 22:41:08 2021 +0100
+++ b/src/Pure/Admin/build_cygwin.scala	Sun Jan 10 13:04:29 2021 +0100
@@ -18,7 +18,7 @@
     mirror: String = default_mirror,
     more_packages: List[String] = Nil)
   {
-    require(Platform.is_windows)
+    require(Platform.is_windows, "Windows platform expected")
 
     Isabelle_System.with_tmp_dir("cygwin")(tmp_dir =>
       {