diff --git a/scrape/rules.go b/scrape/rules.go
index f5b51ee77d..45364c224d 100644
--- a/scrape/rules.go
+++ b/scrape/rules.go
@@ -63,7 +63,7 @@ func (r *ruleEngine) NewScrapeBatch() Batch {
 func (r *ruleEngine) EvaluateRules(b Batch, ts time.Time, sampleMutator labelsMutator) ([]Sample, error) {
 	var (
 		result  []Sample
-		builder labels.ScratchBuilder
+		builder = labels.NewScratchBuilder(0)
 	)
 	for _, rule := range r.rules {
 		queryable := storage.QueryableFunc(func(_, _ int64) (storage.Querier, error) {
diff --git a/scrape/scrape_test.go b/scrape/scrape_test.go
index 55618adf5e..0e09003d82 100644
--- a/scrape/scrape_test.go
+++ b/scrape/scrape_test.go
@@ -5205,7 +5205,13 @@ metric{l1="1", l2="2"} 5`,
 			require.NoError(t, err)
 
 			require.NoError(t, slApp.Commit())
-			require.Equal(t, test.expectedFloats, app.resultFloats)
+			require.Len(t, app.resultFloats, len(test.expectedFloats))
+			for i := range app.resultFloats {
+				expected, actual := test.expectedFloats[i], app.resultFloats[i]
+				require.Equal(t, expected.metric.String(), actual.metric.String())
+				require.Equal(t, expected.t, actual.t)
+				require.Equal(t, expected.f, actual.f)
+			}
 		})
 	}
 }