Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 1506484)
+++ lucene/CHANGES.txt	(working copy)
@@ -84,6 +84,10 @@
   searchAfter exceeds the number of documents in the reader. 
   (Crocket via Shai Erera)
 
+* LUCENE-5129: CategoryAssociationsContaier no longer supports null association
+  values for categories. If you want to index categories without an
+  association, you should add them using FacetFields. (Shai Erera)
+  
 Optimizations
 
 * LUCENE-5088: Added TermFilter to filter docs by a specific term.
Index: lucene/facet/src/java/org/apache/lucene/facet/associations/AssociationsListBuilder.java
===================================================================
--- lucene/facet/src/java/org/apache/lucene/facet/associations/AssociationsListBuilder.java	(revision 1506484)
+++ lucene/facet/src/java/org/apache/lucene/facet/associations/AssociationsListBuilder.java	(working copy)
@@ -54,13 +54,6 @@
       // build per-association key BytesRef
       CategoryAssociation association = associations.getAssociation(cp);
       
-      if (association == null) {
-        // it is ok to set a null association for a category - it's treated as a
-        // regular category in that case.
-        ++idx;
-        continue;
-      }
-
       BytesRef bytes = res.get(association.getCategoryListID());
       if (bytes == null) {
         bytes = new BytesRef(32);
Index: lucene/facet/src/java/org/apache/lucene/facet/associations/CategoryAssociationsContainer.java
===================================================================
--- lucene/facet/src/java/org/apache/lucene/facet/associations/CategoryAssociationsContainer.java	(revision 1506484)
+++ lucene/facet/src/java/org/apache/lucene/facet/associations/CategoryAssociationsContainer.java	(working copy)
@@ -30,11 +30,12 @@
   
   /**
    * Adds the {@link CategoryAssociation} for the given {@link CategoryPath
-   * category}. Overrides any assocation that was previously set. It is ok to
-   * pass {@code null}, in which case the category will be treated as a regular
-   * one (i.e. without association value).
+   * category}. Overrides any assocation that was previously set.
    */
   public void setAssociation(CategoryPath category, CategoryAssociation association) {
+    if (association == null) {
+      throw new IllegalArgumentException("cannot set a null association to a category");
+    }
     categoryAssociations.put(category, association);
   }