Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 1129315)
+++ lucene/CHANGES.txt	(working copy)
@@ -233,6 +233,9 @@
   
   (Mike McCandless, Michael Busch, Simon Willnauer)
 
+* LUCENE-3146: IndexReader.setNorm throws IllegalStateException if the field
+  does not store norms. (Shai Erera, Mike McCandless)
+  
 API Changes
 
 * LUCENE-2302, LUCENE-1458, LUCENE-2111, LUCENE-2514: Terms are no longer
Index: lucene/src/java/org/apache/lucene/index/SegmentReader.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/SegmentReader.java	(revision 1129315)
+++ lucene/src/java/org/apache/lucene/index/SegmentReader.java	(working copy)
@@ -566,8 +566,10 @@
   protected void doSetNorm(int doc, String field, byte value)
           throws IOException {
     SegmentNorms norm = norms.get(field);
-    if (norm == null)                             // not an indexed field
-      return;
+    if (norm == null) {
+      // field does not store norms
+      throw new IllegalStateException("Cannot set norms for field that does not store norms: " + field);
+    }
 
     normsDirty = true;
     norm.copyOnWrite()[doc] = value;                    // set the value
Index: lucene/src/java/org/apache/lucene/index/IndexReader.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/IndexReader.java	(revision 1129315)
+++ lucene/src/java/org/apache/lucene/index/IndexReader.java	(working copy)
@@ -1026,7 +1026,7 @@
    * values when resetting this, one should base the new value upon the old.
    *
    * <b>NOTE:</b> If this field does not store norms, then
-   * this method call will silently do nothing.
+   * this method throws {@link IllegalStateException}.
    *
    * @see #norms(String)
    * @see Similarity#decodeNormValue(byte)
@@ -1037,6 +1037,7 @@
    *  has this index open (<code>write.lock</code> could not
    *  be obtained)
    * @throws IOException if there is a low-level IO error
+   * @throws IllegalStateException if the field does not store norms
    */
   public synchronized  void setNorm(int doc, String field, byte value)
           throws StaleReaderException, CorruptIndexException, LockObtainFailedException, IOException {
Index: lucene/src/java/org/apache/lucene/document/Field.java
===================================================================
--- lucene/src/java/org/apache/lucene/document/Field.java	(revision 1129315)
+++ lucene/src/java/org/apache/lucene/document/Field.java	(working copy)
@@ -104,11 +104,11 @@
      * less memory usage as norms take up one byte of RAM
      * per indexed field for every document in the index,
      * during searching.  Note that once you index a given
-     * field <i>with</i> norms enabled, disabling norms will
+     * field <i>with</i> norms disabled, enabling norms will
      * have no effect.  In other words, for this to have the
-     * above described effect on a field, all instances of
+     * above described effect on a field, one instance of
      * that field must be indexed with NOT_ANALYZED_NO_NORMS
-     * from the beginning. */
+     * at some point. */
     NOT_ANALYZED_NO_NORMS {
       @Override
       public boolean isIndexed()  { return true;  }