Index: trunk/src/org/openstreetmap/josm/data/osm/AbstractPrimitive.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/osm/AbstractPrimitive.java	(revision 8708)
+++ trunk/src/org/openstreetmap/josm/data/osm/AbstractPrimitive.java	(revision 8709)
@@ -480,5 +480,6 @@
     @Override
     public Map<String, String> getKeys() {
-        Map<String, String> result = new HashMap<>(Utils.hashMapInitialCapacity(keys.length / 2));
+        final Map<String, String> result = new HashMap<>(
+                Utils.hashMapInitialCapacity(this.keys == null ? 0 : this.keys.length / 2));
         String[] keys = this.keys;
         if (keys != null) {