Index: /trunk/src/org/openstreetmap/josm/gui/dialogs/validator/ValidatorTreePanel.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/dialogs/validator/ValidatorTreePanel.java	(revision 7269)
+++ /trunk/src/org/openstreetmap/josm/gui/dialogs/validator/ValidatorTreePanel.java	(revision 7270)
@@ -164,5 +164,8 @@
                 } else if (userObject instanceof String) {
                     String msg = (String) userObject;
-                    msg = msg.substring(0, msg.lastIndexOf(" ("));
+                    int index = msg.lastIndexOf(" (");
+                    if (index > 0) {
+                        msg = msg.substring(0, index);
+                    }
                     oldSelectedRows.add(msg);
                 }