Index: trunk/src/org/openstreetmap/josm/data/osm/User.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/osm/User.java	(revision 13099)
+++ trunk/src/org/openstreetmap/josm/data/osm/User.java	(revision 13100)
@@ -159,7 +159,8 @@
      *
      * @param name User name
+     * @throws NullPointerException if name is null
      */
     public void addName(String name) {
-        names.add(name);
+        names.add(Objects.requireNonNull(name, "name"));
     }
 
@@ -170,4 +171,5 @@
      * data files it is unclear which is the up-to-date user name.
      * @param name the preferred user name to set
+     * @throws NullPointerException if name is null
      */
     public void setPreferredName(String name) {
@@ -177,5 +179,5 @@
         final Collection<String> allNames = new LinkedHashSet<>(names);
         names.clear();
-        names.add(name);
+        names.add(Objects.requireNonNull(name, "name"));
         names.addAll(allNames);
     }