Index: src/java/org/apache/lucene/index/SegmentMerger.java
===================================================================
--- src/java/org/apache/lucene/index/SegmentMerger.java	(revision 1127458)
+++ src/java/org/apache/lucene/index/SegmentMerger.java	(working copy)
@@ -113,13 +113,22 @@
     return mergedDocs;
   }
 
+  /**
+   * NOTE: this method creates a compound file out of all the files returned by
+   * info.files(), including the .del and separate norms file (.s*). Those
+   * should not exist in the result .cfs, and therefore should not be passed to
+   * this method.
+   */
   final Collection<String> createCompoundFile(String fileName, final SegmentInfo info)
           throws IOException {
-
     // Now merge all added files
     Collection<String> files = info.files();
     CompoundFileWriter cfsWriter = new CompoundFileWriter(directory, fileName, checkAbort);
     for (String file : files) {
+      assert !IndexFileNames.matchesExtension(file, IndexFileNames.DELETES_EXTENSION) 
+                : ".del file is not allowed in .cfs: " + file;
+      assert !file.substring(file.lastIndexOf('.') + 1).startsWith(IndexFileNames.SEPARATE_NORMS_EXTENSION) 
+                : "separate norms file (.s*) is not allowed in .cfs: " + file;
       cfsWriter.addFile(file);
     }
     
Index: src/test/org/apache/lucene/index/TestSegmentMerger.java
===================================================================
--- src/test/org/apache/lucene/index/TestSegmentMerger.java	(revision 1127458)
+++ src/test/org/apache/lucene/index/TestSegmentMerger.java	(working copy)
@@ -20,7 +20,12 @@
 import org.apache.lucene.util.LuceneTestCase;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.BufferedIndexInput;
+import org.apache.lucene.analysis.MockAnalyzer;
 import org.apache.lucene.document.Document;
+import org.apache.lucene.document.Field;
+import org.apache.lucene.document.Field.Index;
+import org.apache.lucene.document.Field.Store;
+import org.apache.lucene.index.IndexWriterConfig.OpenMode;
 
 import java.io.IOException;
 import java.util.Collection;
@@ -123,5 +128,50 @@
 
     TestSegmentReader.checkNorms(mergedReader);
     mergedReader.close();
-  }    
+  }
+  
+  public void testInvalidFilesToCreateCompound() throws Exception {
+    Directory dir = newDirectory();
+    IndexWriterConfig iwc = newIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(random));
+    IndexWriter w = new IndexWriter(dir, iwc);
+    
+    // Create an index w/ .del file
+    w.addDocument(new Document());
+    Document doc = new Document();
+    doc.add(new Field("c", "test", Store.NO, Index.ANALYZED));
+    w.addDocument(doc);
+    w.commit();
+    w.deleteDocuments(new Term("c", "test"));
+    w.close();
+    
+    // Assert that SM fails if .del exists
+    SegmentMerger sm = new SegmentMerger(dir, 1, "a", null, null, null);
+    try {
+      sm.createCompoundFile("b1", w.segmentInfos.info(0));
+      fail("should not have been able to create a .cfs with .del and .s* files");
+    } catch (AssertionError e) {
+      // expected
+    }
+    
+    // Create an index w/ .s*
+    w = new IndexWriter(dir, new IndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(random)).setOpenMode(OpenMode.CREATE));
+    doc = new Document();
+    doc.add(new Field("c", "test", Store.NO, Index.ANALYZED));
+    w.addDocument(doc);
+    w.close();
+    IndexReader r = IndexReader.open(dir, false);
+    r.setNorm(0, "c", (byte) 1);
+    r.close();
+    
+    // Assert that SM fails if .s* exists
+    try {
+      sm.createCompoundFile("b2", w.segmentInfos.info(0));
+      fail("should not have been able to create a .cfs with .del and .s* files");
+    } catch (AssertionError e) {
+      // expected
+    }
+
+    dir.close();
+  }
+  
 }