Index: trunk/src/org/openstreetmap/josm/data/osm/AbstractPrimitive.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/osm/AbstractPrimitive.java	(revision 15013)
+++ trunk/src/org/openstreetmap/josm/data/osm/AbstractPrimitive.java	(revision 15014)
@@ -15,4 +15,5 @@
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.TimeUnit;
@@ -498,6 +499,6 @@
         int index = 0;
         for (Entry<String, String> entry:keys.entrySet()) {
-            newKeys[index++] = entry.getKey();
-            newKeys[index++] = entry.getValue();
+            newKeys[index++] = Objects.requireNonNull(entry.getKey());
+            newKeys[index++] = Objects.requireNonNull(entry.getValue());
         }
         this.keys = newKeys;