src/Pure/System/cygwin.scala

--- a/src/Pure/System/cygwin.scala	Sat Jan 09 22:41:08 2021 +0100
+++ b/src/Pure/System/cygwin.scala	Sun Jan 10 13:04:29 2021 +0100
@@ -19,7 +19,7 @@
 
   def init(isabelle_root: String, cygwin_root: String)
   {
-    require(Platform.is_windows)
+    require(Platform.is_windows, "Windows platform expected")
 
     def exec(cmdline: String*)
     {