diff --git a/core/src/main/java/org/elasticsearch/common/lucene/search/function/FiltersFunctionScoreQuery.java b/core/src/main/java/org/elasticsearch/common/lucene/search/function/FiltersFunctionScoreQuery.java
index 314135db661..210b32d5e42 100644
--- a/core/src/main/java/org/elasticsearch/common/lucene/search/function/FiltersFunctionScoreQuery.java
+++ b/core/src/main/java/org/elasticsearch/common/lucene/search/function/FiltersFunctionScoreQuery.java
@@ -225,7 +225,8 @@ public class FiltersFunctionScoreQuery extends Query {
             }
 
             FiltersFunctionFactorScorer scorer = (FiltersFunctionFactorScorer)scorer(context);
-            scorer.advance(doc);
+            int actualDoc = scorer.advance(doc);
+            assert (actualDoc == doc);
             double score = scorer.computeScore(doc, subQueryExpl.getValue());
             Explanation factorExplanation = Explanation.match(
                     CombineFunction.toFloat(score),