Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 948421)
+++ CHANGES.txt	(working copy)
@@ -406,6 +406,9 @@
   lock (previously we only released on IOException).  (Tamas Cservenak
   via Mike McCandless)
 
+* LUCENE-2478: Fix CachingWrapperFilter to not throw NPE when
+  Filter.getDocIdSet() returns null.  (Uwe Schindler, Daniel Noll)
+
 New features
 
 * LUCENE-2128: Parallelized fetching document frequencies during weight
Index: src/java/org/apache/lucene/search/CachingWrapperFilter.java
===================================================================
--- src/java/org/apache/lucene/search/CachingWrapperFilter.java	(revision 948421)
+++ src/java/org/apache/lucene/search/CachingWrapperFilter.java	(working copy)
@@ -53,7 +53,10 @@
    *  an {@link OpenBitSetDISI}.
    */
   protected DocIdSet docIdSetToCache(DocIdSet docIdSet, IndexReader reader) throws IOException {
-    if (docIdSet.isCacheable()) {
+    if (docIdSet == null) {
+      // this is better than returning null, as the nonnull result can be cached
+      return DocIdSet.EMPTY_DOCIDSET;
+    } else if (docIdSet.isCacheable()) {
       return docIdSet;
     } else {
       final DocIdSetIterator it = docIdSet.iterator();
Index: src/test/org/apache/lucene/search/TestCachingWrapperFilter.java
===================================================================
--- src/test/org/apache/lucene/search/TestCachingWrapperFilter.java	(revision 948421)
+++ src/test/org/apache/lucene/search/TestCachingWrapperFilter.java	(working copy)
@@ -57,6 +57,55 @@
     reader.close();
   }
   
+  public void testNullDocIdSet() throws Exception {
+    Directory dir = new RAMDirectory();
+    IndexWriter writer = new IndexWriter(dir, new IndexWriterConfig(
+        TEST_VERSION_CURRENT, new MockAnalyzer()));
+    writer.close();
+
+    IndexReader reader = IndexReader.open(dir, true);
+
+    final Filter filter = new Filter() {
+      @Override
+      public DocIdSet getDocIdSet(IndexReader reader) {
+        return null;
+      }
+    };
+    CachingWrapperFilter cacher = new CachingWrapperFilter(filter);
+
+    // the caching filter should return the empty set constant
+    assertSame(DocIdSet.EMPTY_DOCIDSET, cacher.getDocIdSet(reader));
+    
+    reader.close();
+  }
+  
+  public void testNullDocIdSetIterator() throws Exception {
+    Directory dir = new RAMDirectory();
+    IndexWriter writer = new IndexWriter(dir, new IndexWriterConfig(
+        TEST_VERSION_CURRENT, new MockAnalyzer()));
+    writer.close();
+
+    IndexReader reader = IndexReader.open(dir, true);
+
+    final Filter filter = new Filter() {
+      @Override
+      public DocIdSet getDocIdSet(IndexReader reader) {
+        return new DocIdSet() {
+          @Override
+          public DocIdSetIterator iterator() {
+            return null;
+          }
+        };
+      }
+    };
+    CachingWrapperFilter cacher = new CachingWrapperFilter(filter);
+
+    // the caching filter should return the empty set constant
+    assertSame(DocIdSet.EMPTY_DOCIDSET, cacher.getDocIdSet(reader));
+    
+    reader.close();
+  }
+  
   private static void assertDocIdSetCacheable(IndexReader reader, Filter filter, boolean shouldCacheable) throws IOException {
     final CachingWrapperFilter cacher = new CachingWrapperFilter(filter);
     final DocIdSet originalSet = filter.getDocIdSet(reader);