Index: src/test/org/apache/lucene/index/TestMultiReader.java
===================================================================
--- src/test/org/apache/lucene/index/TestMultiReader.java	(revision 354923)
+++ src/test/org/apache/lucene/index/TestMultiReader.java	(working copy)
@@ -69,6 +69,18 @@
     assertTrue(vector != null);
     TestSegmentReader.checkNorms(reader);
   }
+
+  public void testUndeleteAll() throws IOException {
+    sis.read(dir);
+    MultiReader reader = new MultiReader(dir, sis, false, readers);
+    assertTrue(reader != null);
+    assertEquals( 2, reader.numDocs() );
+    reader.delete(0);
+    assertEquals( 1, reader.numDocs() );
+    reader.undeleteAll();
+    assertEquals( 2, reader.numDocs() );
+  }
+        
   
   public void testTermVectors() {
     MultiReader reader = new MultiReader(dir, sis, false, readers);
Index: src/java/org/apache/lucene/index/MultiReader.java
===================================================================
--- src/java/org/apache/lucene/index/MultiReader.java	(revision 354923)
+++ src/java/org/apache/lucene/index/MultiReader.java	(working copy)
@@ -122,6 +122,7 @@
     for (int i = 0; i < subReaders.length; i++)
       subReaders[i].undeleteAll();
     hasDeletions = false;
+    numDocs = -1;      // invalidate cache
   }
 
   private int readerIndex(int n) {    // find reader for doc n: