Index: lucene/core/src/test/org/apache/lucene/index/TestIndexWriterConfig.java
===================================================================
--- lucene/core/src/test/org/apache/lucene/index/TestIndexWriterConfig.java	(révision 1456384)
+++ lucene/core/src/test/org/apache/lucene/index/TestIndexWriterConfig.java	(copie de travail)
@@ -324,8 +324,12 @@
     assertEquals(TieredMergePolicy.class, conf.getMergePolicy().getClass());
     conf.setMergePolicy(new LogDocMergePolicy());
     assertEquals(LogDocMergePolicy.class, conf.getMergePolicy().getClass());
-    conf.setMergePolicy(null);
-    assertEquals(LogByteSizeMergePolicy.class, conf.getMergePolicy().getClass());
+    try {
+      conf.setMergePolicy(null);
+      assertFalse(true);
+    } catch (NullPointerException e) {
+      // ok
+    }
   }
 
   public void testLiveChangeToCFS() throws Exception {
Index: lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java
===================================================================
--- lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java	(révision 1456384)
+++ lucene/core/src/java/org/apache/lucene/index/IndexWriterConfig.java	(copie de travail)
@@ -272,12 +272,14 @@
    * Expert: {@link MergePolicy} is invoked whenever there are changes to the
    * segments in the index. Its role is to select which merges to do, if any,
    * and return a {@link MergePolicy.MergeSpecification} describing the merges.
-   * It also selects merges to do for forceMerge. (The default is
-   * {@link LogByteSizeMergePolicy}.
+   * It also selects merges to do for forceMerge.
    *
    * <p>Only takes effect when IndexWriter is first created. */
   public IndexWriterConfig setMergePolicy(MergePolicy mergePolicy) {
-    this.mergePolicy = mergePolicy == null ? new LogByteSizeMergePolicy() : mergePolicy;
+    if (mergePolicy == null) {
+      throw new NullPointerException();
+    }
+    this.mergePolicy = mergePolicy;
     return this;
   }