Index: trunk/src/org/openstreetmap/josm/data/imagery/ImageryInfo.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/imagery/ImageryInfo.java	(revision 16100)
+++ trunk/src/org/openstreetmap/josm/data/imagery/ImageryInfo.java	(revision 16101)
@@ -19,5 +19,4 @@
 import java.util.Set;
 import java.util.TreeSet;
-import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 import java.util.regex.Matcher;
@@ -305,7 +304,7 @@
     private boolean isGeoreferenceValid;
     /** which layers should be activated by default on layer addition. **/
-    private List<DefaultLayer> defaultLayers = new ArrayList<>();
+    private List<DefaultLayer> defaultLayers = Collections.emptyList();
     /** HTTP headers **/
-    private Map<String, String> customHttpHeaders = new ConcurrentHashMap<>();
+    private Map<String, String> customHttpHeaders = Collections.emptyMap();
     /** Should this map be transparent **/
     private boolean transparent = true;
@@ -588,5 +587,5 @@
             }
         }
-        customHttpHeaders = e.customHttpHeaders;
+        setCustomHttpHeaders(e.customHttpHeaders);
         transparent = e.transparent;
         minimumTileExpire = e.minimumTileExpire;
@@ -635,6 +634,6 @@
         this.icon = intern(i.icon);
         this.isGeoreferenceValid = i.isGeoreferenceValid;
-        this.defaultLayers = i.defaultLayers;
-        this.customHttpHeaders = i.customHttpHeaders;
+        setDefaultLayers(i.defaultLayers);
+        setCustomHttpHeaders(i.customHttpHeaders);
         this.transparent = i.transparent;
         this.minimumTileExpire = i.minimumTileExpire;
@@ -1571,5 +1570,5 @@
      */
     public void setDefaultLayers(List<DefaultLayer> layers) {
-        this.defaultLayers = layers;
+        this.defaultLayers = Utils.toUnmodifiableList(layers);
     }
 
@@ -1579,7 +1578,4 @@
      */
     public Map<String, String> getCustomHttpHeaders() {
-        if (customHttpHeaders == null) {
-            return Collections.emptyMap();
-        }
         return customHttpHeaders;
     }
@@ -1590,5 +1586,5 @@
      */
     public void setCustomHttpHeaders(Map<String, String> customHttpHeaders) {
-        this.customHttpHeaders = customHttpHeaders;
+        this.customHttpHeaders = Utils.toUnmodifiableMap(customHttpHeaders);
     }