diff --git a/storage/local/series.go b/storage/local/series.go
index 0dcad859a..2c49dc9a5 100644
--- a/storage/local/series.go
+++ b/storage/local/series.go
@@ -313,21 +313,27 @@ func (s *memorySeries) dropChunks(t model.Time) error {
 			break
 		}
 	}
-	if keepIdx > 0 {
-		s.chunkDescs = append(
-			make([]*chunkDesc, 0, len(s.chunkDescs)-keepIdx),
-			s.chunkDescs[keepIdx:]...,
-		)
-		s.persistWatermark -= keepIdx
-		if s.persistWatermark < 0 {
-			panic("dropped unpersisted chunks from memory")
-		}
-		if s.chunkDescsOffset != -1 {
-			s.chunkDescsOffset += keepIdx
-		}
-		numMemChunkDescs.Sub(float64(keepIdx))
-		s.dirty = true
+	if keepIdx == len(s.chunkDescs) && !s.headChunkClosed {
+		// Never drop an open head chunk.
+		keepIdx--
 	}
+	if keepIdx <= 0 {
+		// Nothing to drop.
+		return nil
+	}
+	s.chunkDescs = append(
+		make([]*chunkDesc, 0, len(s.chunkDescs)-keepIdx),
+		s.chunkDescs[keepIdx:]...,
+	)
+	s.persistWatermark -= keepIdx
+	if s.persistWatermark < 0 {
+		panic("dropped unpersisted chunks from memory")
+	}
+	if s.chunkDescsOffset != -1 {
+		s.chunkDescsOffset += keepIdx
+	}
+	numMemChunkDescs.Sub(float64(keepIdx))
+	s.dirty = true
 	return nil
 }
 
diff --git a/storage/local/series_test.go b/storage/local/series_test.go
new file mode 100644
index 000000000..86467c270
--- /dev/null
+++ b/storage/local/series_test.go
@@ -0,0 +1,63 @@
+// Copyright 2016 The Prometheus Authors
+// Licensed 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.
+
+package local
+
+import (
+	"testing"
+	"time"
+
+	"github.com/prometheus/common/model"
+)
+
+func TestDropChunks(t *testing.T) {
+	s, err := newMemorySeries(nil, nil, time.Time{})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	s.add(model.SamplePair{
+		Timestamp: 100,
+		Value:     42,
+	})
+	s.add(model.SamplePair{
+		Timestamp: 110,
+		Value:     4711,
+	})
+
+	err = s.dropChunks(110)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(s.chunkDescs) == 0 {
+		t.Fatal("chunk dropped too early")
+	}
+
+	err = s.dropChunks(115)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(s.chunkDescs) == 0 {
+		t.Fatal("open head chunk dropped")
+	}
+
+	s.headChunkClosed = true
+	s.persistWatermark = 1
+	err = s.dropChunks(115)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(s.chunkDescs) != 0 {
+		t.Error("did not drop closed head chunk")
+	}
+}