Index: lucene/src/java/org/apache/lucene/search/CachingWrapperFilter.java
===================================================================
--- lucene/src/java/org/apache/lucene/search/CachingWrapperFilter.java	(revision 1213973)
+++ lucene/src/java/org/apache/lucene/search/CachingWrapperFilter.java	(working copy)
@@ -37,29 +37,51 @@
   // level of the readers hierarchy it should be cached.
   Filter filter;
 
-  protected final FilterCache<DocIdSet> cache;
+  protected final FilterCache cache = new FilterCache();
   private final boolean recacheDeletes;
 
-  private static class FilterCache<T> {
+  // helper for adding an identity reference as key in WeakHashMap
+  static final class IdentityKeyWrapper<T> {
+    public final T key;
+    private final int hashCode;
 
-    /**
-     * A transient Filter cache (package private because of test)
-     */
-    private final Map<Object,Map<Object,T>> cache = new WeakHashMap<Object,Map<Object,T>>();
+    public IdentityKeyWrapper(T key) {
+      this.key = key;
+      this.hashCode = System.identityHashCode(key);
+    }
 
-    public synchronized T get(IndexReader reader, Object coreKey, Object coreSubKey) throws IOException {
-      Map<Object,T> innerCache = cache.get(coreKey);
+    public int hashCode() {
+      return hashCode;
+    }
+
+    @SuppressWarnings({"unchecked", "rawtypes"})
+    public boolean equals(Object o) {
+      if (o == this) return true;
+      if (o instanceof IdentityKeyWrapper) {
+        return ((IdentityKeyWrapper<T>) o).key == this.key;
+      }
+      return false;
+    }
+
+  }
+
+  private static class FilterCache {
+    private final Map<Object,Map<IdentityKeyWrapper<Bits>,DocIdSet>> cache = new WeakHashMap<Object,Map<IdentityKeyWrapper<Bits>,DocIdSet>>();
+
+    public synchronized DocIdSet get(IndexReader reader, Object coreKey, Bits coreSubKey) throws IOException {
+      Map<IdentityKeyWrapper<Bits>,DocIdSet> innerCache = cache.get(coreKey);
       if (innerCache == null) {
-        innerCache = new WeakHashMap<Object,T>();
+        innerCache = new WeakHashMap<IdentityKeyWrapper<Bits>,DocIdSet>();
         cache.put(coreKey, innerCache);
       }
 
-      return innerCache.get(coreSubKey);
+      return innerCache.get(new IdentityKeyWrapper<Bits>(coreSubKey));
     }
 
-    public synchronized void put(Object coreKey, Object coreSubKey, T value) {
-      cache.get(coreKey).put(coreSubKey, value);
+    public synchronized void put(Object coreKey, Bits coreSubKey, DocIdSet value) {
+      cache.get(coreKey).put(new IdentityKeyWrapper<Bits>(coreSubKey), value);
     }
+    
   }
 
   /** Wraps another filter's result and caches it.
@@ -79,7 +101,6 @@
   public CachingWrapperFilter(Filter filter, boolean recacheDeletes) {
     this.filter = filter;
     this.recacheDeletes = recacheDeletes;
-    cache = new FilterCache<DocIdSet>();
   }
 
   /** Provide the DocIdSet to be cached, using the DocIdSet provided