diff --git a/testing/agent/src/main/java/sbt/FrameworkWrapper.java b/testing/agent/src/main/java/sbt/FrameworkWrapper.java
index 21b9cc4b5..640546db4 100644
--- a/testing/agent/src/main/java/sbt/FrameworkWrapper.java
+++ b/testing/agent/src/main/java/sbt/FrameworkWrapper.java
@@ -151,7 +151,7 @@ class EventWrapper implements Event {
 	}
 
 	public long duration() {
-		return 0;  // Just return 0 as old event does not have duration.
+		return -1;  // Just return -1 as old event does not have duration.
 	}
 }
 
@@ -259,4 +259,4 @@ class RunnerWrapper implements Runner {
 	public String[] remoteArgs() {
 		return new String[0];  // Old framework does not support remoteArgs
 	}
-}
\ No newline at end of file
+}