Index: /trunk/src/org/openstreetmap/josm/gui/dialogs/properties/PropertiesDialog.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/dialogs/properties/PropertiesDialog.java	(revision 5326)
+++ /trunk/src/org/openstreetmap/josm/gui/dialogs/properties/PropertiesDialog.java	(revision 5327)
@@ -1485,5 +1485,6 @@
         @Override
         protected Collection<String> getString(OsmPrimitive p, String key) {
-            return Collections.singleton(p.get(key));
+            String v = p.get(key);
+            return v == null ? null : Collections.singleton(v);
         }
     }