diff --git a/compile/src/main/scala/sbt/compiler/CompilerArguments.scala b/compile/src/main/scala/sbt/compiler/CompilerArguments.scala
index 295bae5bb..df279bb37 100644
--- a/compile/src/main/scala/sbt/compiler/CompilerArguments.scala
+++ b/compile/src/main/scala/sbt/compiler/CompilerArguments.scala
@@ -63,7 +63,7 @@ final class CompilerArguments(scalaInstance: xsbti.compile.ScalaInstance, cp: xs
   def bootClasspathFor(classpath: Seq[File]) = bootClasspath(hasLibrary(classpath))
 
   import Path._
-  def extClasspath: Seq[File] = (IO.parseClasspath(System.getProperty("java.ext.dirs")) * "*.jar").get
+  def extClasspath: Seq[File] = (IO.parseClasspath(System.getProperty("java.ext.dirs", "")) * "*.jar").get
 }
 object CompilerArguments {
   val BootClasspathOption = "-bootclasspath"