diff --git src/org/openstreetmap/josm/data/cache/CacheEntryAttributes.java src/org/openstreetmap/josm/data/cache/CacheEntryAttributes.java
index 5d4ae08..b792c94 100644
--- src/org/openstreetmap/josm/data/cache/CacheEntryAttributes.java
+++ src/org/openstreetmap/josm/data/cache/CacheEntryAttributes.java
@@ -2,7 +2,6 @@
 package org.openstreetmap.josm.data.cache;
 
 import java.util.HashMap;
-import java.util.Map;
 
 import org.apache.commons.jcs.engine.ElementAttributes;
 
@@ -14,7 +13,7 @@ import org.apache.commons.jcs.engine.ElementAttributes;
  */
 public class CacheEntryAttributes extends ElementAttributes {
     private static final long serialVersionUID = 1L; //version
-    private transient Map<String, String> attrs = new HashMap<String, String>();
+    private HashMap<String, String> attrs = new HashMap<String, String>(); // type needs to be HashMap to guarantee serialization and silence Sonar
     private final static String NO_TILE_AT_ZOOM = "noTileAtZoom";
     private final static String ETAG = "Etag";
     private final static String LAST_MODIFICATION = "lastModification";