diff --git a/compiler/src/dotty/tools/dotc/fromtasty/Debug.scala b/compiler/src/dotty/tools/dotc/fromtasty/Debug.scala
index c249f00a2368..f2d9adca1e50 100644
--- a/compiler/src/dotty/tools/dotc/fromtasty/Debug.scala
+++ b/compiler/src/dotty/tools/dotc/fromtasty/Debug.scala
@@ -6,6 +6,7 @@ import scala.util.control.NonFatal
 
 import dotty.tools.io.Directory
 
+import java.io.{File => JFile}
 import java.nio.file.{Files, Paths}
 
 object Debug {
@@ -65,7 +66,7 @@ object Debug {
 
   private def insertClasspathInArgs(args: List[String], cp: String): List[String] = {
     val (beforeCp, fromCp) = args.span(_ != "-classpath")
-    val classpath = fromCp.drop(1).headOption.fold(cp)(_ + ":" + cp)
+    val classpath = fromCp.drop(1).headOption.fold(cp)(_ + JFile.pathSeparator + cp)
     "-classpath" :: classpath :: beforeCp ::: fromCp.drop(2)
   }
 }