Index: /trunk/src/org/openstreetmap/josm/data/Preferences.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/Preferences.java	(revision 6655)
+++ /trunk/src/org/openstreetmap/josm/data/Preferences.java	(revision 6656)
@@ -1039,5 +1039,7 @@
             Main.info("Defaults for " + key + " differ: " + def + " != " + defaultsMap.get(key));
         }
-        defaultsMap.put(key, def.copy());
+        if (def.getValue() != null || oldDef == null) {
+            defaultsMap.put(key, def.copy());
+        }
         Setting prop = settingsMap.get(key);
         if (klass.isInstance(prop)) {