diff --git a/db.go b/db.go
index 80a335c17..17f3e5f5f 100644
--- a/db.go
+++ b/db.go
@@ -80,6 +80,7 @@ type Appender interface {
 	// Returned reference numbers are ephemeral and may be rejected in calls
 	// to AddFast() at any point. Adding the sample via Add() returns a new
 	// reference number.
+	// If the reference is the empty string it must not be used for caching.
 	Add(l labels.Labels, t int64, v float64) (string, error)
 
 	// Add adds a sample pair for the referenced series. It is generally faster
@@ -616,6 +617,9 @@ func (a *dbAppender) Add(lset labels.Labels, t int64, v float64) (string, error)
 	}
 	a.samples++
 
+	if ref == "" {
+		return "", nil
+	}
 	return string(append(h.meta.ULID[:], ref...)), nil
 }
 
diff --git a/head.go b/head.go
index d64c4eda5..90e03f727 100644
--- a/head.go
+++ b/head.go
@@ -450,7 +450,7 @@ func (a *headAppender) Add(lset labels.Labels, t int64, v float64) (string, erro
 		// XXX(fabxc): there's no fast path for multiple samples for the same new series
 		// in the same transaction. We always return the invalid empty ref. It's has not
 		// been a relevant use case so far and is not worth the trouble.
-		return nullRef, a.AddFast(string(refb), t, v)
+		return "", a.AddFast(string(refb), t, v)
 	}
 
 	// The series is completely new.
@@ -471,11 +471,9 @@ func (a *headAppender) Add(lset labels.Labels, t int64, v float64) (string, erro
 	a.newHashes[hash] = ref
 	binary.BigEndian.PutUint64(refb, ref)
 
-	return nullRef, a.AddFast(string(refb), t, v)
+	return "", a.AddFast(string(refb), t, v)
 }
 
-var nullRef = string([]byte{0, 0, 0, 0, 0, 0, 0, 0})
-
 func (a *headAppender) AddFast(ref string, t int64, v float64) error {
 	if len(ref) != 8 {
 		return errors.Wrap(ErrNotFound, "invalid ref length")