Index: lucene/CHANGES.txt
===================================================================
--- lucene/CHANGES.txt	(revision 1491165)
+++ lucene/CHANGES.txt	(working copy)
@@ -141,6 +141,8 @@
 * LUCENE-5033: SlowFuzzyQuery was accepting too many terms (documents) when
   provided minSimilarity is an int > 1 (Tim Allison via Mike McCandless)
 
+* LUCENE-5045: DrillSideways.search did not work on an empty index. (Shai Erera)
+
 Optimizations
 
 * LUCENE-4936: Improve numeric doc values compression in case all values share
Index: lucene/facet/src/java/org/apache/lucene/facet/search/DrillSideways.java
===================================================================
--- lucene/facet/src/java/org/apache/lucene/facet/search/DrillSideways.java	(revision 1491165)
+++ lucene/facet/src/java/org/apache/lucene/facet/search/DrillSideways.java	(working copy)
@@ -402,8 +402,13 @@
       query = new DrillDownQuery(filter, query);
     }
     if (sort != null) {
+      int limit = searcher.getIndexReader().maxDoc();
+      if (limit == 0) {
+        limit = 1; // the collector does not alow numHits = 0
+      }
+      topN = Math.min(topN, limit);
       final TopFieldCollector hitCollector = TopFieldCollector.create(sort,
-                                                                      Math.min(topN, searcher.getIndexReader().maxDoc()),
+                                                                      topN,
                                                                       after,
                                                                       true,
                                                                       doDocScores,
@@ -422,7 +427,12 @@
    */
   public DrillSidewaysResult search(ScoreDoc after,
                                     DrillDownQuery query, int topN, FacetSearchParams fsp) throws IOException {
-    TopScoreDocCollector hitCollector = TopScoreDocCollector.create(Math.min(topN, searcher.getIndexReader().maxDoc()), after, true);
+    int limit = searcher.getIndexReader().maxDoc();
+    if (limit == 0) {
+      limit = 1; // the collector does not alow numHits = 0
+    }
+    topN = Math.min(topN, limit);
+    TopScoreDocCollector hitCollector = TopScoreDocCollector.create(topN, after, true);
     DrillSidewaysResult r = search(query, hitCollector, fsp);
     return new DrillSidewaysResult(r.facetResults, hitCollector.topDocs());
   }
Index: lucene/facet/src/test/org/apache/lucene/facet/search/TestDrillSideways.java
===================================================================
--- lucene/facet/src/test/org/apache/lucene/facet/search/TestDrillSideways.java	(revision 1491165)
+++ lucene/facet/src/test/org/apache/lucene/facet/search/TestDrillSideways.java	(working copy)
@@ -59,15 +59,18 @@
 import org.apache.lucene.search.Scorer;
 import org.apache.lucene.search.Sort;
 import org.apache.lucene.search.SortField;
+import org.apache.lucene.search.SortField.Type;
 import org.apache.lucene.search.TermQuery;
 import org.apache.lucene.search.TopDocs;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.FixedBitSet;
+import org.apache.lucene.util.IOUtils;
 import org.apache.lucene.util.InPlaceMergeSorter;
 import org.apache.lucene.util.InfoStream;
 import org.apache.lucene.util._TestUtil;
+import org.junit.Test;
 
 public class TestDrillSideways extends FacetTestCase {
 
@@ -1144,5 +1147,34 @@
     }
     return b.toString();
   }
+  
+  @Test
+  public void testEmptyIndex() throws Exception {
+    // LUCENE-5045: make sure DrillSideways works with an empty index
+    Directory dir = newDirectory();
+    Directory taxoDir = newDirectory();
+    writer = new RandomIndexWriter(random(), dir);
+    taxoWriter = new DirectoryTaxonomyWriter(taxoDir, IndexWriterConfig.OpenMode.CREATE);
+    IndexSearcher searcher = newSearcher(writer.getReader());
+    writer.close();
+    TaxonomyReader taxoReader = new DirectoryTaxonomyReader(taxoWriter);
+    taxoWriter.close();
+
+    // Count "Author"
+    FacetSearchParams fsp = new FacetSearchParams(new CountFacetRequest(new CategoryPath("Author"), 10));
+
+    DrillSideways ds = new DrillSideways(searcher, taxoReader);
+    DrillDownQuery ddq = new DrillDownQuery(fsp.indexingParams, new MatchAllDocsQuery());
+    ddq.add(new CategoryPath("Author", "Lisa"));
+    
+    DrillSidewaysResult r = ds.search(null, ddq, 10, fsp); // this used to fail on IllegalArgEx
+    assertEquals(0, r.hits.totalHits);
+
+    r = ds.search(ddq, null, null, 10, new Sort(new SortField("foo", Type.INT)), false, false, fsp); // this used to fail on IllegalArgEx
+    assertEquals(0, r.hits.totalHits);
+    
+    IOUtils.close(searcher.getIndexReader(), taxoReader, dir, taxoDir);
+  }
+  
 }