Index: trunk/src/org/openstreetmap/josm/tools/AlphanumComparator.java
===================================================================
--- trunk/src/org/openstreetmap/josm/tools/AlphanumComparator.java	(revision 5086)
+++ trunk/src/org/openstreetmap/josm/tools/AlphanumComparator.java	(revision 5087)
@@ -71,4 +71,12 @@
     @Override
     public int compare(String s1, String s2) {
+        if (s1 == null && s2 == null) {
+            return 0;
+        } else if (s1 == null) {
+            return -1;
+        } else if (s2 == null) {
+            return 1;
+        }
+
         int thisMarker = 0;
         int thatMarker = 0;