Index: trunk/src/org/openstreetmap/josm/data/validation/tests/TagChecker.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/validation/tests/TagChecker.java	(revision 14731)
+++ trunk/src/org/openstreetmap/josm/data/validation/tests/TagChecker.java	(revision 14732)
@@ -631,5 +631,7 @@
         if (presetValues != null)
             sets.add(presetValues);
-        sets.add(oftenUsedTags.get(key));
+        Set<String> usedValues = oftenUsedTags.get(key);
+        if (usedValues != null)
+            sets.add(usedValues);
         for (Set<String> possibleValues: sets) {
             if (possibleValues.contains(harmonizedValue)) {