diff --git a/src/Pure/System/mingw.scala b/src/Pure/System/mingw.scala
--- a/src/Pure/System/mingw.scala
+++ b/src/Pure/System/mingw.scala
@@ -1,51 +1,51 @@
 /*  Title:      Pure/System/mingw.scala
     Author:     Makarius
 
 Support for MSYS2/MinGW64 on Windows.
 */
 
 package isabelle
 
 
 object MinGW
 {
   def environment: List[String] =
     List("PATH=/usr/bin:/bin:/mingw64/bin", "CONFIG_SITE=/mingw64/etc/config.site")
 
   def environment_export: String =
     environment.map(a => "export " + Bash.string(a)).mkString("", "\n", "\n")
 
   val none: MinGW = new MinGW(None)
   def apply(path: Path) = new MinGW(Some(path))
 }
 
 class MinGW private(val root: Option[Path])
 {
   override def toString: String =
     root match {
       case None => "MinGW.none"
       case Some(msys_root) => "MinGW(" + msys_root.toString + ")"
     }
 
   def bash_script(script: String): String =
     root match {
       case None => script
       case Some(msys_root) =>
         File.bash_path(msys_root + Path.explode("usr/bin/bash")) +
           " -c " + Bash.string(MinGW.environment_export + script)
     }
 
   def get_root: Path =
     if (!Platform.is_windows) error("Windows platform required")
     else if (root.isEmpty) error("Windows platform requires msys/mingw root specification")
     else root.get
 
   def check
   {
     if (Platform.is_windows) {
       get_root
       try { require(Isabelle_System.bash(bash_script("uname -s")).check.out.startsWith("MSYS")) }
-      catch { case ERROR(_) => error("Bad msys/mingw installation " + get_root) }
+      catch { case ERROR(msg) => cat_error("Bad msys/mingw installation " + get_root, msg) }
     }
   }
 }