diff --git a/remoting/src/main/java/hudson/remoting/Which.java b/remoting/src/main/java/hudson/remoting/Which.java
index abe0722508f976d35f3984d228e3d46add8fbc3c..ab19e3bb8801822b7dd2fdb26a41d460851159d8 100644
--- a/remoting/src/main/java/hudson/remoting/Which.java
+++ b/remoting/src/main/java/hudson/remoting/Which.java
@@ -36,6 +36,7 @@ public class Which {
             int n = clazz.getName().split("\\.").length; // how many slashes do wo need to cut?
             for( ; n>0; n-- ) {
                 int idx = Math.max(resURL.lastIndexOf('/'), resURL.lastIndexOf('\\'));
+                if(idx<0)   throw new IllegalArgumentException(resURL);
                 resURL = resURL.substring(0,idx);
             }