diff --git a/main/actions/src/main/scala/sbt/ForkTests.scala b/main/actions/src/main/scala/sbt/ForkTests.scala
index 981c714cf..b9186b1fe 100755
--- a/main/actions/src/main/scala/sbt/ForkTests.scala
+++ b/main/actions/src/main/scala/sbt/ForkTests.scala
@@ -49,6 +49,8 @@ private[sbt] object ForkTests
 							case _: java.net.SocketException => return
 						}
 					val os = new ObjectOutputStream(socket.getOutputStream)
+					// Must flush the header that the constructor writes, otherwise the ObjectInputStream on the other end may block indefinitely
+					os.flush()
 					val is = new ObjectInputStream(socket.getInputStream)
 
 					try {
diff --git a/testing/agent/src/main/java/sbt/ForkMain.java b/testing/agent/src/main/java/sbt/ForkMain.java
index c54d177da..a494adbc4 100755
--- a/testing/agent/src/main/java/sbt/ForkMain.java
+++ b/testing/agent/src/main/java/sbt/ForkMain.java
@@ -92,6 +92,8 @@ public class ForkMain {
 		Socket socket = new Socket(InetAddress.getByName(null), Integer.valueOf(args[0]));
 		final ObjectInputStream is = new ObjectInputStream(socket.getInputStream());
 		final ObjectOutputStream os = new ObjectOutputStream(socket.getOutputStream());
+		// Must flush the header that the constructor writes, otherwise the ObjectInputStream on the other end may block indefinitely
+		os.flush();
 		try {
 			try {
 				new Run().run(is, os);