Index: lucene/core/src/java/org/apache/lucene/search/FilteredDocIdSet.java
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- lucene/core/src/java/org/apache/lucene/search/FilteredDocIdSet.java	(revision b8d4f36992bd147fdcd410be8ae1fef093995f8e)
+++ lucene/core/src/java/org/apache/lucene/search/FilteredDocIdSet.java	(revision )
@@ -69,7 +69,11 @@
    */
   @Override
   public DocIdSetIterator iterator() throws IOException {
-    return new FilteredDocIdSetIterator(_innerSet.iterator()) {
+    DocIdSetIterator iterator = _innerSet.iterator();
+    if (iterator == null) {
+      return null;
+    }
+    return new FilteredDocIdSetIterator(iterator) {
       @Override
       protected boolean match(int docid) throws IOException {
         return FilteredDocIdSet.this.match(docid);