Index: trunk/src/org/openstreetmap/josm/spi/preferences/MapListSetting.java
===================================================================
--- trunk/src/org/openstreetmap/josm/spi/preferences/MapListSetting.java	(revision 14826)
+++ trunk/src/org/openstreetmap/josm/spi/preferences/MapListSetting.java	(revision 14827)
@@ -7,4 +7,5 @@
 import java.util.List;
 import java.util.Map;
+import java.util.SortedMap;
 
 /**
@@ -41,5 +42,5 @@
             throw new IllegalArgumentException("Error: Null as list element in preference setting");
         for (Map<String, String> map : value) {
-            if (map.containsKey(null))
+            if (!(map instanceof SortedMap) && map.containsKey(null))
                 throw new IllegalArgumentException("Error: Null as map key in preference setting");
             if (map.containsValue(null))