diff --git a/pkg/value/value.go b/pkg/value/value.go
index df847db6d..d0b4942af 100644
--- a/pkg/value/value.go
+++ b/pkg/value/value.go
@@ -13,7 +13,15 @@
 
 package value
 
-var (
+import (
+	"math"
+)
+
+const (
 	NormalNaN uint64 = 0x7ff8000000000001 // A quiet NaN. This is also math.NaN().
 	StaleNaN  uint64 = 0x7ff4000000000000 // A signalling NaN, starting 01 to allow for expansion.
 )
+
+func IsStaleNaN(v float64) bool {
+	return math.Float64bits(v) == StaleNaN
+}
diff --git a/promql/engine.go b/promql/engine.go
index 9ea3ab3f0..5fbc2dd1d 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -757,7 +757,7 @@ func (ev *evaluator) vectorSelector(node *VectorSelector) Vector {
 				continue
 			}
 		}
-		if math.Float64bits(v) == value.StaleNaN {
+		if value.IsStaleNaN(v) {
 			continue
 		}
 		vec = append(vec, Sample{
@@ -830,7 +830,7 @@ func (ev *evaluator) matrixSelector(node *MatrixSelector) Matrix {
 		buf := it.Buffer()
 		for buf.Next() {
 			t, v := buf.At()
-			if math.Float64bits(v) == value.StaleNaN {
+			if value.IsStaleNaN(v) {
 				continue
 			}
 			// Values in the buffer are guaranteed to be smaller than maxt.
@@ -840,7 +840,7 @@ func (ev *evaluator) matrixSelector(node *MatrixSelector) Matrix {
 		}
 		// The seeked sample might also be in the range.
 		t, v = it.Values()
-		if t == maxt && math.Float64bits(v) != value.StaleNaN {
+		if t == maxt && !value.IsStaleNaN(v) {
 			allPoints = append(allPoints, Point{T: t, V: v})
 		}
 
diff --git a/retrieval/scrape.go b/retrieval/scrape.go
index 4cdcf5a28..ad89b6a85 100644
--- a/retrieval/scrape.go
+++ b/retrieval/scrape.go
@@ -423,6 +423,7 @@ type scrapeLoop struct {
 	appender       func() storage.Appender
 	reportAppender func() storage.Appender
 
+	// TODO: Keep only the values from the last scrape to avoid a memory leak.
 	refCache                map[string]uint64         // Parsed string to ref.
 	lsetCache               map[uint64]lsetCacheEntry // Ref to labelset and string
 	samplesInPreviousScrape map[string]labels.Labels