diff --git a/storage/fanout_test.go b/storage/fanout_test.go
index cc839b6ed..a5d04ea8f 100644
--- a/storage/fanout_test.go
+++ b/storage/fanout_test.go
@@ -195,10 +195,18 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 			}
 			qs = append(qs, tc.extraQueriers...)
 
-			merged, _, _ := NewMergeQuerier(qs[0], qs, ChainedSeriesMerge).Select(false, nil)
-			for merged.Next() {
+			mergedQuerier, _, _ := NewMergeQuerier(qs[0], qs, ChainedSeriesMerge).Select(false, nil)
+
+			// Get all merged series upfront to make sure there are no incorrectly retained shared
+			// buffers causing bugs.
+			var mergedSeries []Series
+			for mergedQuerier.Next() {
+				mergedSeries = append(mergedSeries, mergedQuerier.At())
+			}
+			testutil.Ok(t, mergedQuerier.Err())
+
+			for _, actualSeries := range mergedSeries {
 				testutil.Assert(t, tc.expected.Next(), "Expected Next() to be true")
-				actualSeries := merged.At()
 				expectedSeries := tc.expected.At()
 				testutil.Equals(t, expectedSeries.Labels(), actualSeries.Labels())
 
@@ -207,7 +215,6 @@ func TestMergeQuerierWithChainMerger(t *testing.T) {
 				testutil.Equals(t, expErr, actErr)
 				testutil.Equals(t, expSmpl, actSmpl)
 			}
-			testutil.Ok(t, merged.Err())
 			testutil.Assert(t, !tc.expected.Next(), "Expected Next() to be false")
 		})
 	}
diff --git a/storage/generic.go b/storage/generic.go
index a5a694cb2..d23e51a6c 100644
--- a/storage/generic.go
+++ b/storage/generic.go
@@ -108,26 +108,24 @@ func (q *chunkQuerierAdapter) Select(sortSeries bool, hints *SelectHints, matche
 
 type seriesMergerAdapter struct {
 	VerticalSeriesMergeFunc
-	buf []Series
 }
 
 func (a *seriesMergerAdapter) Merge(s ...Labels) Labels {
-	a.buf = a.buf[:0]
+	buf := make([]Series, 0, len(s))
 	for _, ser := range s {
-		a.buf = append(a.buf, ser.(Series))
+		buf = append(buf, ser.(Series))
 	}
-	return a.VerticalSeriesMergeFunc(a.buf...)
+	return a.VerticalSeriesMergeFunc(buf...)
 }
 
 type chunkSeriesMergerAdapter struct {
 	VerticalChunkSeriesMergerFunc
-	buf []ChunkSeries
 }
 
 func (a *chunkSeriesMergerAdapter) Merge(s ...Labels) Labels {
-	a.buf = a.buf[:0]
+	buf := make([]ChunkSeries, 0, len(s))
 	for _, ser := range s {
-		a.buf = append(a.buf, ser.(ChunkSeries))
+		buf = append(buf, ser.(ChunkSeries))
 	}
-	return a.VerticalChunkSeriesMergerFunc(a.buf...)
+	return a.VerticalChunkSeriesMergerFunc(buf...)
 }