diff --git a/storage/fanout.go b/storage/fanout.go
index 0f94c2974..b44bf34fe 100644
--- a/storage/fanout.go
+++ b/storage/fanout.go
@@ -246,7 +246,8 @@ func (q *mergeQuerier) Close() error {
 type mergeSeriesSet struct {
 	currentLabels labels.Labels
 	currentSets   []SeriesSet
-	sets          seriesSetHeap
+	heap          seriesSetHeap
+	sets          []SeriesSet
 }
 
 func newMergeSeriesSet(sets []SeriesSet) SeriesSet {
@@ -259,7 +260,8 @@ func newMergeSeriesSet(sets []SeriesSet) SeriesSet {
 		}
 	}
 	return &mergeSeriesSet{
-		sets: h,
+		heap: h,
+		sets: sets,
 	}
 }
 
@@ -268,18 +270,18 @@ func (c *mergeSeriesSet) Next() bool {
 	// we can drop them, otherwise they should be inserted back into the heap.
 	for _, set := range c.currentSets {
 		if set.Next() {
-			heap.Push(&c.sets, set)
+			heap.Push(&c.heap, set)
 		}
 	}
-	if len(c.sets) == 0 {
+	if len(c.heap) == 0 {
 		return false
 	}
 
 	// Now, pop items of the heap that have equal label sets.
 	c.currentSets = nil
-	c.currentLabels = c.sets[0].At().Labels()
-	for len(c.sets) > 0 && labels.Equal(c.currentLabels, c.sets[0].At().Labels()) {
-		set := heap.Pop(&c.sets).(SeriesSet)
+	c.currentLabels = c.heap[0].At().Labels()
+	for len(c.heap) > 0 && labels.Equal(c.currentLabels, c.heap[0].At().Labels()) {
+		set := heap.Pop(&c.heap).(SeriesSet)
 		c.currentSets = append(c.currentSets, set)
 	}
 	return true