diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 16f02b5137d..a9f01ae2dc6 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -83,6 +83,9 @@ Bug Fixes
   romaji even for out-of-vocabulary kana cases (e.g. half-width forms).
   (Robert Muir)
 
+* LUCENE-4504: Fix broken sort comparator in ValueSource.getSortField,
+  used when sorting by a function query.  (Tom Shally via Robert Muir)
+
 Optimizations
 
 * LUCENE-4443: Lucene41PostingsFormat no longer writes unnecessary offsets 
diff --git a/lucene/queries/src/java/org/apache/lucene/queries/function/ValueSource.java b/lucene/queries/src/java/org/apache/lucene/queries/function/ValueSource.java
index 91c94cf4f9c..67b41cbd803 100644
--- a/lucene/queries/src/java/org/apache/lucene/queries/function/ValueSource.java
+++ b/lucene/queries/src/java/org/apache/lucene/queries/function/ValueSource.java
@@ -192,7 +192,7 @@ public abstract class ValueSource {
       if (docValue < value) {
         return -1;
       } else if (docValue > value) {
-        return -1;
+        return 1;
       } else {
         return 0;
       }
diff --git a/lucene/queries/src/test/org/apache/lucene/queries/function/TestFunctionQuerySort.java b/lucene/queries/src/test/org/apache/lucene/queries/function/TestFunctionQuerySort.java
new file mode 100644
index 00000000000..65cf830fa7d
--- /dev/null
+++ b/lucene/queries/src/test/org/apache/lucene/queries/function/TestFunctionQuerySort.java
@@ -0,0 +1,99 @@
+package org.apache.lucene.queries.function;
+
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import java.io.IOException;
+import org.apache.lucene.document.Document;
+import org.apache.lucene.document.Field;
+import org.apache.lucene.document.StringField;
+import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.IndexWriterConfig;
+import org.apache.lucene.index.RandomIndexWriter;
+import org.apache.lucene.queries.function.valuesource.IntFieldSource;
+import org.apache.lucene.search.FieldDoc;
+import org.apache.lucene.search.IndexSearcher;
+import org.apache.lucene.search.MatchAllDocsQuery;
+import org.apache.lucene.search.Query;
+import org.apache.lucene.search.ScoreDoc;
+import org.apache.lucene.search.Sort;
+import org.apache.lucene.search.SortField;
+import org.apache.lucene.search.TopDocs;
+import org.apache.lucene.store.Directory;
+import org.apache.lucene.util.LuceneTestCase;
+
+/** Test that functionquery's getSortField() actually works */
+public class TestFunctionQuerySort extends LuceneTestCase {
+
+  public void testSearchAfterWhenSortingByFunctionValues() throws IOException {
+    Directory dir = newDirectory();
+    IndexWriterConfig iwc = newIndexWriterConfig(TEST_VERSION_CURRENT, null);
+    iwc.setMergePolicy(newLogMergePolicy()); // depends on docid order
+    RandomIndexWriter writer = new RandomIndexWriter(random(), dir, iwc);
+
+    Document doc = new Document();
+    Field field = new StringField("value", "", Field.Store.YES);
+    doc.add(field);
+
+    // Save docs unsorted (decreasing value n, n-1, ...)
+    final int NUM_VALS = 5;
+    for (int val = NUM_VALS; val > 0; val--) {
+      field.setStringValue(Integer.toString(val));
+      writer.addDocument(doc);
+    }
+
+    // Open index
+    IndexReader reader = writer.getReader();
+    writer.close();
+    IndexSearcher searcher = new IndexSearcher(reader);
+
+    // Get ValueSource from FieldCache
+    IntFieldSource src = new IntFieldSource("value");
+    // ...and make it a sort criterion
+    SortField sf = src.getSortField(false).rewrite(searcher);
+    Sort orderBy = new Sort(sf);
+
+    // Get hits sorted by our FunctionValues (ascending values)
+    Query q = new MatchAllDocsQuery();
+    TopDocs hits = searcher.search(q, Integer.MAX_VALUE, orderBy);
+    assertEquals(NUM_VALS, hits.scoreDocs.length);
+    // Verify that sorting works in general
+    int i = 0;
+    for (ScoreDoc hit : hits.scoreDocs) {
+      int valueFromDoc = Integer.parseInt(reader.document(hit.doc).get("value"));
+      assertEquals(++i, valueFromDoc);
+    }
+
+    // Now get hits after hit #2 using IS.searchAfter()
+    int afterIdx = 1;
+    FieldDoc afterHit = (FieldDoc) hits.scoreDocs[afterIdx];
+    hits = searcher.searchAfter(afterHit, q, Integer.MAX_VALUE, orderBy);
+
+    // Expected # of hits: NUM_VALS - 2
+    assertEquals(NUM_VALS - (afterIdx + 1), hits.scoreDocs.length);
+
+    // Verify that hits are actually "after"
+    int afterValue = ((Double) afterHit.fields[0]).intValue();
+    for (ScoreDoc hit : hits.scoreDocs) {
+      int val = Integer.parseInt(reader.document(hit.doc).get("value"));
+      assertTrue(afterValue <= val);
+      assertFalse(hit.doc == afterHit.doc);
+    }
+    reader.close();
+    dir.close();
+  }
+}