Index: /trunk/src/org/openstreetmap/josm/data/osm/WaySegment.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/osm/WaySegment.java	(revision 13029)
+++ /trunk/src/org/openstreetmap/josm/data/osm/WaySegment.java	(revision 13030)
@@ -25,8 +25,12 @@
      * @param w The way
      * @param i The node lower index
+     * @throws IllegalArgumentException in case of invalid index
      */
     public WaySegment(Way w, int i) {
         way = w;
         lowerIndex = i;
+        if (i < 0 || i >= w.getNodesCount() - 1) {
+            throw new IllegalArgumentException(toString());
+        }
     }