Index: src/org/openstreetmap/josm/spi/preferences/MapListSetting.java
===================================================================
--- src/org/openstreetmap/josm/spi/preferences/MapListSetting.java	(revision 14786)
+++ src/org/openstreetmap/josm/spi/preferences/MapListSetting.java	(working copy)
@@ -6,6 +6,7 @@
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.SortedMap;
 
 /**
  * Setting containing a {@link List} of {@link Map}s of {@link String} values.
@@ -40,7 +41,7 @@
         if (value.contains(null))
             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))
                 throw new IllegalArgumentException("Error: Null as map value in preference setting");