diff --git a/db.go b/db.go
index 2491ce145..010a5968d 100644
--- a/db.go
+++ b/db.go
@@ -261,7 +261,7 @@ func OpenShard(path string, i int, logger log.Logger) (*Shard, error) {
 	s := &Shard{
 		path:      path,
 		logger:    logger,
-		metrics:   newShardMetrics(prometheus.DefaultRegisterer, i),
+		metrics:   newShardMetrics(nil, i),
 		heads:     heads,
 		persisted: persisted,
 		cutc:      make(chan struct{}, 1),
@@ -335,7 +335,7 @@ func (s *Shard) appendBatch(samples []hashedSample) error {
 	}
 
 	// TODO(fabxc): randomize over time and use better scoring function.
-	if head.bstats.SampleCount/(uint64(head.bstats.ChunkCount)+1) > 400 {
+	if head.bstats.SampleCount/(uint64(head.bstats.ChunkCount)+1) > 500 {
 		if err := s.cut(); err != nil {
 			s.logger.Log("msg", "cut failed", "err", err)
 		}
diff --git a/querier.go b/querier.go
index 4cacc7631..f6383b186 100644
--- a/querier.go
+++ b/querier.go
@@ -381,7 +381,7 @@ func (s *shardSeriesSet) compare() int {
 	if s.bdone {
 		return -1
 	}
-	return labels.Compare(s.a.At().Labels(), s.a.At().Labels())
+	return labels.Compare(s.a.At().Labels(), s.b.At().Labels())
 }
 
 func (s *shardSeriesSet) Next() bool {
diff --git a/querier_test.go b/querier_test.go
index 1ad093bb7..842f8c73f 100644
--- a/querier_test.go
+++ b/querier_test.go
@@ -139,6 +139,57 @@ func TestShardSeriesSet(t *testing.T) {
 				}),
 			}),
 		},
+		{
+			a: newListSeriesSet([]Series{
+				newSeries(map[string]string{
+					"handler":  "prometheus",
+					"instance": "127.0.0.1:9090",
+				}, []sample{
+					{t: 1, v: 1},
+				}),
+				newSeries(map[string]string{
+					"handler":  "prometheus",
+					"instance": "localhost:9090",
+				}, []sample{
+					{t: 1, v: 2},
+				}),
+			}),
+			b: newListSeriesSet([]Series{
+				newSeries(map[string]string{
+					"handler":  "prometheus",
+					"instance": "127.0.0.1:9090",
+				}, []sample{
+					{t: 2, v: 1},
+				}),
+				newSeries(map[string]string{
+					"handler":  "query",
+					"instance": "localhost:9090",
+				}, []sample{
+					{t: 2, v: 2},
+				}),
+			}),
+			exp: newListSeriesSet([]Series{
+				newSeries(map[string]string{
+					"handler":  "prometheus",
+					"instance": "127.0.0.1:9090",
+				}, []sample{
+					{t: 1, v: 1},
+					{t: 2, v: 1},
+				}),
+				newSeries(map[string]string{
+					"handler":  "prometheus",
+					"instance": "localhost:9090",
+				}, []sample{
+					{t: 1, v: 2},
+				}),
+				newSeries(map[string]string{
+					"handler":  "query",
+					"instance": "localhost:9090",
+				}, []sample{
+					{t: 2, v: 2},
+				}),
+			}),
+		},
 	}
 
 Outer: