diff --git a/prometheus/gauge_test.go b/prometheus/gauge_test.go
index a2e3c14..b70da7d 100644
--- a/prometheus/gauge_test.go
+++ b/prometheus/gauge_test.go
@@ -139,7 +139,7 @@ func TestGaugeVecConcurrency(t *testing.T) {
 				start.Wait()
 				for i, v := range vals {
 					sStreams[pick[i]] <- v
-					gge.WithLabelValues(string('A' + pick[i])).Add(v)
+					gge.WithLabelValues(string('A' + rune(pick[i]))).Add(v)
 				}
 				end.Done()
 			}(vals)
@@ -147,7 +147,7 @@ func TestGaugeVecConcurrency(t *testing.T) {
 		start.Done()
 
 		for i := range sStreams {
-			if expected, got := <-results[i], math.Float64frombits(gge.WithLabelValues(string('A'+i)).(*gauge).valBits); math.Abs(expected-got) > 0.000001 {
+			if expected, got := <-results[i], math.Float64frombits(gge.WithLabelValues(string('A'+rune(i))).(*gauge).valBits); math.Abs(expected-got) > 0.000001 {
 				t.Fatalf("expected approx. %f, got %f", expected, got)
 				return false
 			}
diff --git a/prometheus/histogram_test.go b/prometheus/histogram_test.go
index 0b4826c..e779c1a 100644
--- a/prometheus/histogram_test.go
+++ b/prometheus/histogram_test.go
@@ -279,7 +279,7 @@ func TestHistogramVecConcurrency(t *testing.T) {
 			go func(vals []float64) {
 				start.Wait()
 				for i, v := range vals {
-					his.WithLabelValues(string('A' + picks[i])).Observe(v)
+					his.WithLabelValues(string('A' + rune(picks[i]))).Observe(v)
 				}
 				end.Done()
 			}(vals)
@@ -292,7 +292,7 @@ func TestHistogramVecConcurrency(t *testing.T) {
 
 		for i := 0; i < vecLength; i++ {
 			m := &dto.Metric{}
-			s := his.WithLabelValues(string('A' + i))
+			s := his.WithLabelValues(string('A' + rune(i)))
 			s.(Histogram).Write(m)
 
 			if got, want := len(m.Histogram.Bucket), len(testBuckets)-1; got != want {
diff --git a/prometheus/summary_test.go b/prometheus/summary_test.go
index 43b8396..eff733a 100644
--- a/prometheus/summary_test.go
+++ b/prometheus/summary_test.go
@@ -321,7 +321,7 @@ func TestSummaryVecConcurrency(t *testing.T) {
 			go func(vals []float64) {
 				start.Wait()
 				for i, v := range vals {
-					sum.WithLabelValues(string('A' + picks[i])).Observe(v)
+					sum.WithLabelValues(string('A' + rune(picks[i]))).Observe(v)
 				}
 				end.Done()
 			}(vals)
@@ -334,7 +334,7 @@ func TestSummaryVecConcurrency(t *testing.T) {
 
 		for i := 0; i < vecLength; i++ {
 			m := &dto.Metric{}
-			s := sum.WithLabelValues(string('A' + i))
+			s := sum.WithLabelValues(string('A' + rune(i)))
 			s.(Summary).Write(m)
 			if got, want := int(*m.Summary.SampleCount), len(allVars[i]); got != want {
 				t.Errorf("got sample count %d for label %c, want %d", got, 'A'+i, want)