Index: lucene/facet/src/java/org/apache/lucene/facet/index/CountingListBuilder.java
===================================================================
--- lucene/facet/src/java/org/apache/lucene/facet/index/CountingListBuilder.java	(revision 1460965)
+++ lucene/facet/src/java/org/apache/lucene/facet/index/CountingListBuilder.java	(working copy)
@@ -152,13 +152,16 @@
       if (op != OrdinalPolicy.NO_PARENTS) {
         // need to add parents too
         int parent = taxoWriter.getParent(ordinal);
-        while (parent > 0) {
-          ordinals.ints[ordinals.length++] = parent;
-          parent = taxoWriter.getParent(parent);
+        if (parent > 0) {
+          // only do this if the category is not a dimension itself, otherwise, it was just discarded by the 'if' below
+          while (parent > 0) {
+            ordinals.ints[ordinals.length++] = parent;
+            parent = taxoWriter.getParent(parent);
+          }
+          if (op == OrdinalPolicy.ALL_BUT_DIMENSION) { // discard the last added parent, which is the dimension
+            ordinals.length--;
+          }
         }
-        if (op == OrdinalPolicy.ALL_BUT_DIMENSION) { // discard the last added parent, which is the dimension
-          ordinals.length--;
-        }
       }
     }
     return ordinalsEncoder.encode(ordinals);
Index: lucene/facet/src/java/org/apache/lucene/facet/search/FacetsAccumulator.java
===================================================================
--- lucene/facet/src/java/org/apache/lucene/facet/search/FacetsAccumulator.java	(revision 1460965)
+++ lucene/facet/src/java/org/apache/lucene/facet/search/FacetsAccumulator.java	(working copy)
@@ -182,10 +182,12 @@
         continue;
       }
       CategoryListParams clp = searchParams.indexingParams.getCategoryListParams(fr.categoryPath);
-      OrdinalPolicy ordinalPolicy = clp .getOrdinalPolicy(fr.categoryPath.components[0]);
-      if (ordinalPolicy == OrdinalPolicy.NO_PARENTS) {
-        // rollup values
-        aggregator.rollupValues(fr, rootOrd, children, siblings, facetArrays);
+      if (fr.categoryPath.length > 0) { // someone might ask to aggregate the ROOT category
+        OrdinalPolicy ordinalPolicy = clp.getOrdinalPolicy(fr.categoryPath.components[0]);
+        if (ordinalPolicy == OrdinalPolicy.NO_PARENTS) {
+          // rollup values
+          aggregator.rollupValues(fr, rootOrd, children, siblings, facetArrays);
+        }
       }
       
       FacetResultsHandler frh = createFacetResultsHandler(fr);
Index: lucene/facet/src/test/org/apache/lucene/facet/search/TestFacetsCollector.java
===================================================================
--- lucene/facet/src/test/org/apache/lucene/facet/search/TestFacetsCollector.java	(revision 1460965)
+++ lucene/facet/src/test/org/apache/lucene/facet/search/TestFacetsCollector.java	(working copy)
@@ -1,6 +1,7 @@
 package org.apache.lucene.facet.search;
 
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
@@ -190,5 +191,41 @@
     
     IOUtils.close(taxo, taxoDir, r, indexDir);
   }
+  
+  @Test
+  public void testCountRoot() throws Exception {
+    // LUCENE-4882: FacetsAccumulator threw NPE if a FacetRequest was defined on CP.EMPTY
+    Directory indexDir = newDirectory();
+    Directory taxoDir = newDirectory();
+    
+    TaxonomyWriter taxonomyWriter = new DirectoryTaxonomyWriter(taxoDir);
+    IndexWriter iw = new IndexWriter(indexDir, newIndexWriterConfig(TEST_VERSION_CURRENT, new MockAnalyzer(random())));
+    
+    FacetFields facetFields = new FacetFields(taxonomyWriter);
+    for(int i = atLeast(30); i > 0; --i) {
+      Document doc = new Document();
+      facetFields.addFields(doc, Arrays.asList(new CategoryPath("a"), new CategoryPath("b")));
+      iw.addDocument(doc);
+    }
+    
+    taxonomyWriter.close();
+    iw.close();
+    
+    DirectoryReader r = DirectoryReader.open(indexDir);
+    DirectoryTaxonomyReader taxo = new DirectoryTaxonomyReader(taxoDir);
+    
+    FacetSearchParams fsp = new FacetSearchParams(new CountFacetRequest(CategoryPath.EMPTY, 10));
+    
+    final FacetsAccumulator fa = random().nextBoolean() ? new FacetsAccumulator(fsp, r, taxo) : new StandardFacetsAccumulator(fsp, r, taxo);
+    FacetsCollector fc = FacetsCollector.create(fa);
+    new IndexSearcher(r).search(new MatchAllDocsQuery(), fc);
+    
+    FacetResult res = fc.getFacetResults().get(0);
+    for (FacetResultNode node : res.getFacetResultNode().subResults) {
+      assertEquals(r.numDocs(), (int) node.value);
+    }
+    
+    IOUtils.close(taxo, taxoDir, r, indexDir);
+  }
 
 }