Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 1064132)
+++ lucene/CHANGES.txt	(working copy)
@@ -371,6 +371,9 @@
 * LUCENE-1846: DateTools now uses the US locale everywhere, so DateTools.round()
   is safe also in strange locales.  (Uwe Schindler)
 
+* LUCENE-2891: IndexWriterConfig did not accept -1 in setReaderTermIndexDivisor,
+  which can be used to prevent loading the terms index into memory. (Shai Erera)
+
 New features
 
 * LUCENE-2128: Parallelized fetching document frequencies during weight
Index: lucene/src/test/org/apache/lucene/index/TestIndexWriterConfig.java
===================================================================
--- lucene/src/test/org/apache/lucene/index/TestIndexWriterConfig.java	(revision 1064132)
+++ lucene/src/test/org/apache/lucene/index/TestIndexWriterConfig.java	(working copy)
@@ -230,6 +230,23 @@
       // this is expected
     }
 
+    // Test setReaderTermsIndexDivisor
+    try {
+      conf.setReaderTermsIndexDivisor(0);
+      fail("should not have succeeded to set termsIndexDivisor to 0");
+    } catch (IllegalArgumentException e) {
+      // this is expected
+    }
+    
+    // Setting to -1 is ok
+    conf.setReaderTermsIndexDivisor(-1);
+    try {
+      conf.setReaderTermsIndexDivisor(-2);
+      fail("should not have succeeded to set termsIndexDivisor to < -1");
+    } catch (IllegalArgumentException e) {
+      // this is expected
+    }
+    
     assertEquals(IndexWriterConfig.DEFAULT_MAX_THREAD_STATES, conf.getMaxThreadStates());
     conf.setMaxThreadStates(5);
     assertEquals(5, conf.getMaxThreadStates());
Index: lucene/src/test/org/apache/lucene/index/TestIndexWriterReader.java
===================================================================
--- lucene/src/test/org/apache/lucene/index/TestIndexWriterReader.java	(revision 1064132)
+++ lucene/src/test/org/apache/lucene/index/TestIndexWriterReader.java	(working copy)
@@ -970,4 +970,26 @@
     dir.close();
     assertTrue(didWarm.get());
   }
+  
+  public void testNoTermsIndex() throws Exception {
+    Directory dir = newDirectory();
+    IndexWriter w = new IndexWriter(dir, newIndexWriterConfig(
+        TEST_VERSION_CURRENT, new WhitespaceAnalyzer(TEST_VERSION_CURRENT))
+        .setReaderTermsIndexDivisor(-1));
+    Document doc = new Document();
+    doc.add(new Field("f", "val", Store.NO, Index.ANALYZED));
+    w.addDocument(doc);
+    IndexReader r = IndexReader.open(w);
+    try {
+      r.termDocs(new Term("f", "val"));
+      fail("should have failed to seek by term, since term index was not loaded");
+    } catch (IllegalStateException e) {
+      // expected - we didn't load the term index
+    } finally {
+      r.close();
+      w.close();
+      dir.close();
+    }
+  }
+  
 }
Index: lucene/src/java/org/apache/lucene/index/IndexReader.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/IndexReader.java	(revision 1064132)
+++ lucene/src/java/org/apache/lucene/index/IndexReader.java	(working copy)
@@ -405,7 +405,10 @@
    *  memory.  By setting this to a value > 1 you can reduce
    *  memory usage, at the expense of higher latency when
    *  loading a TermInfo.  The default value is 1.  Set this
-   *  to -1 to skip loading the terms index entirely.
+   *  to -1 to skip loading the terms index entirely. This is only useful in 
+   *  advanced situations when you will only .next() through all terms; 
+   *  attempts to seek will hit an exception.
+   *  
    * @throws CorruptIndexException if the index is corrupt
    * @throws IOException if there is a low-level IO error
    */
Index: lucene/src/java/org/apache/lucene/index/IndexWriterConfig.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/IndexWriterConfig.java	(revision 1064132)
+++ lucene/src/java/org/apache/lucene/index/IndexWriterConfig.java	(working copy)
@@ -539,10 +539,13 @@
   /** Sets the term index divisor passed to any readers that
    *  IndexWriter opens, for example when apply deletes or
    *  creating a near-real-time reader in {@link
-   *  IndexWriter#getReader}. */
+   *  IndexWriter#getReader}. If you pass -1, the terms index 
+   *  won't be loaded by the readers. This is only useful in 
+   *  advanced situations when you will only .next() through 
+   *  all terms; attempts to seek will hit an exception . */
   public IndexWriterConfig setReaderTermsIndexDivisor(int divisor) {
-    if (divisor <= 0) {
-      throw new IllegalArgumentException("divisor must be >= 1 (got " + divisor + ")");
+    if (divisor <= 0 && divisor != -1) {
+      throw new IllegalArgumentException("divisor must be >= 1, or -1 (got " + divisor + ")");
     }
     readerTermsIndexDivisor = divisor;
     return this;