diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index af675ba23d3..b916043820d 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -65,6 +65,9 @@ Bug Fixes
   toString() was changed to better reflect the reader structure.
   (Mike McCandless, Uwe Schindler)
 
+* LUCENE-4959: Fix incorrect return value in
+  SimpleNaiveBayesClassifier.assignClass. (Alexey Kutin via Adrien Grand)
+
 Optimizations
 
 * LUCENE-4938: Don't use an unnecessarily large priority queue in IndexSearcher
diff --git a/lucene/classification/src/java/org/apache/lucene/classification/SimpleNaiveBayesClassifier.java b/lucene/classification/src/java/org/apache/lucene/classification/SimpleNaiveBayesClassifier.java
index 6617b139b37..692fe4f8492 100644
--- a/lucene/classification/src/java/org/apache/lucene/classification/SimpleNaiveBayesClassifier.java
+++ b/lucene/classification/src/java/org/apache/lucene/classification/SimpleNaiveBayesClassifier.java
@@ -117,7 +117,7 @@ public class SimpleNaiveBayesClassifier implements Classifier<BytesRef> {
       double clVal = calculatePrior(next) * calculateLikelihood(tokenizedDoc, next);
       if (clVal > max) {
         max = clVal;
-        foundClass = next.clone();
+        foundClass = BytesRef.deepCopyOf(next);
       }
     }
     return new ClassificationResult<BytesRef>(foundClass, max);
diff --git a/lucene/classification/src/test/org/apache/lucene/classification/ClassificationTestBase.java b/lucene/classification/src/test/org/apache/lucene/classification/ClassificationTestBase.java
index 2225603f6d2..3b08e43fb2f 100644
--- a/lucene/classification/src/test/org/apache/lucene/classification/ClassificationTestBase.java
+++ b/lucene/classification/src/test/org/apache/lucene/classification/ClassificationTestBase.java
@@ -24,6 +24,7 @@ import org.apache.lucene.document.TextField;
 import org.apache.lucene.index.RandomIndexWriter;
 import org.apache.lucene.index.SlowCompositeReaderWrapper;
 import org.apache.lucene.store.Directory;
+import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.LuceneTestCase;
 import org.junit.After;
 import org.junit.Before;
@@ -32,6 +33,11 @@ import org.junit.Before;
  * Base class for testing {@link Classifier}s
  */
 public abstract class ClassificationTestBase<T> extends LuceneTestCase {
+  public final static String POLITICS_INPUT = "Here are some interesting questions and answers about Mitt Romney.. If you don't know the answer to the question about Mitt Romney, then simply click on the answer below the question section.";
+  public static final BytesRef POLITICS_RESULT = new BytesRef("politics");
+
+  public static final String TECHNOLOGY_INPUT = "Much is made of what the likes of Facebook, Google and Apple know about users. Truth is, Amazon may know more.";
+  public static final BytesRef TECHNOLOGY_RESULT = new BytesRef("technology");
 
   private RandomIndexWriter indexWriter;
   private String textFieldName;
@@ -59,14 +65,13 @@ public abstract class ClassificationTestBase<T> extends LuceneTestCase {
   }
 
 
-  protected void checkCorrectClassification(Classifier<T> classifier, T expectedResult, Analyzer analyzer, String classFieldName) throws Exception {
+  protected void checkCorrectClassification(Classifier<T> classifier, String inputDoc, T expectedResult, Analyzer analyzer, String classFieldName) throws Exception {
     SlowCompositeReaderWrapper compositeReaderWrapper = null;
     try {
       populateIndex(analyzer);
       compositeReaderWrapper = new SlowCompositeReaderWrapper(indexWriter.getReader());
       classifier.train(compositeReaderWrapper, textFieldName, classFieldName, analyzer);
-      String newText = "Much is made of what the likes of Facebook, Google and Apple know about users. Truth is, Amazon may know more.";
-      ClassificationResult<T> classificationResult = classifier.assignClass(newText);
+      ClassificationResult<T> classificationResult = classifier.assignClass(inputDoc);
       assertNotNull(classificationResult.getAssignedClass());
       assertEquals("got an assigned class of " + classificationResult.getAssignedClass(), expectedResult, classificationResult.getAssignedClass());
       assertTrue("got a not positive score " + classificationResult.getScore(), classificationResult.getScore() > 0);
diff --git a/lucene/classification/src/test/org/apache/lucene/classification/KNearestNeighborClassifierTest.java b/lucene/classification/src/test/org/apache/lucene/classification/KNearestNeighborClassifierTest.java
index c80f45a90a5..2e2b066576c 100644
--- a/lucene/classification/src/test/org/apache/lucene/classification/KNearestNeighborClassifierTest.java
+++ b/lucene/classification/src/test/org/apache/lucene/classification/KNearestNeighborClassifierTest.java
@@ -27,7 +27,7 @@ public class KNearestNeighborClassifierTest extends ClassificationTestBase<Bytes
 
   @Test
   public void testBasicUsage() throws Exception {
-     checkCorrectClassification(new KNearestNeighborClassifier(1), new BytesRef("technology"), new MockAnalyzer(random()), categoryFieldName);
+    checkCorrectClassification(new KNearestNeighborClassifier(1), TECHNOLOGY_INPUT, TECHNOLOGY_RESULT, new MockAnalyzer(random()), categoryFieldName);
   }
 
 }
diff --git a/lucene/classification/src/test/org/apache/lucene/classification/SimpleNaiveBayesClassifierTest.java b/lucene/classification/src/test/org/apache/lucene/classification/SimpleNaiveBayesClassifierTest.java
index 40341587b42..46890c47293 100644
--- a/lucene/classification/src/test/org/apache/lucene/classification/SimpleNaiveBayesClassifierTest.java
+++ b/lucene/classification/src/test/org/apache/lucene/classification/SimpleNaiveBayesClassifierTest.java
@@ -34,12 +34,13 @@ public class SimpleNaiveBayesClassifierTest extends ClassificationTestBase<Bytes
 
   @Test
   public void testBasicUsage() throws Exception {
-    checkCorrectClassification(new SimpleNaiveBayesClassifier(), new BytesRef("technology"), new MockAnalyzer(random()), categoryFieldName);
+    checkCorrectClassification(new SimpleNaiveBayesClassifier(), TECHNOLOGY_INPUT, TECHNOLOGY_RESULT, new MockAnalyzer(random()), categoryFieldName);
+    checkCorrectClassification(new SimpleNaiveBayesClassifier(), POLITICS_INPUT, POLITICS_RESULT, new MockAnalyzer(random()), categoryFieldName);
   }
 
   @Test
   public void testNGramUsage() throws Exception {
-    checkCorrectClassification(new SimpleNaiveBayesClassifier(), new BytesRef("technology"), new NGramAnalyzer(), categoryFieldName);
+    checkCorrectClassification(new SimpleNaiveBayesClassifier(), TECHNOLOGY_INPUT, TECHNOLOGY_RESULT, new NGramAnalyzer(), categoryFieldName);
   }
 
   private class NGramAnalyzer extends Analyzer {