diff --git a/promql/quantile.go b/promql/quantile.go
index 6e63efab6..a25f8917c 100644
--- a/promql/quantile.go
+++ b/promql/quantile.go
@@ -168,7 +168,7 @@ func coalesceBuckets(buckets buckets) buckets {
 
 func ensureMonotonic(buckets buckets) {
 	max := buckets[0].count
-	for i := range buckets[1:] {
+	for i := 1; i < len(buckets); i++ {
 		switch {
 		case buckets[i].count > max:
 			max = buckets[i].count
diff --git a/promql/testdata/histograms.test b/promql/testdata/histograms.test
index b7ec13682..ec236576b 100644
--- a/promql/testdata/histograms.test
+++ b/promql/testdata/histograms.test
@@ -151,21 +151,25 @@ eval instant at 50m histogram_quantile(0.5, rate(request_duration_seconds_bucket
 
 # A histogram with nonmonotonic bucket counts. This may happen when recording
 # rule evaluation or federation races scrape ingestion, causing some buckets
-# counts to be derived from fewer samples. The wrong answer we want to avoid
-# is for histogram_quantile(0.99, nonmonotonic_bucket) to return ~1000 instead
-# of 1.
+# counts to be derived from fewer samples.
 
 load 5m
-    nonmonotonic_bucket{le="0.1"}   0+1x10
-    nonmonotonic_bucket{le="1"}     0+9x10
-    nonmonotonic_bucket{le="10"}    0+8x10
-    nonmonotonic_bucket{le="100"}   0+8x10
+    nonmonotonic_bucket{le="0.1"}   0+2x10
+    nonmonotonic_bucket{le="1"}     0+1x10
+    nonmonotonic_bucket{le="10"}    0+5x10
+    nonmonotonic_bucket{le="100"}   0+4x10
     nonmonotonic_bucket{le="1000"}  0+9x10
-    nonmonotonic_bucket{le="+Inf"}  0+9x10
+    nonmonotonic_bucket{le="+Inf"}  0+8x10
 
 # Nonmonotonic buckets
+eval instant at 50m histogram_quantile(0.01, nonmonotonic_bucket)
+    {} 0.0045
+
+eval instant at 50m histogram_quantile(0.5, nonmonotonic_bucket)
+    {} 8.5
+
 eval instant at 50m histogram_quantile(0.99, nonmonotonic_bucket)
-    {} 0.989875
+    {} 979.75
 
 # Buckets with different representations of the same upper bound.
 eval instant at 50m histogram_quantile(0.5, rate(mixed_bucket[5m]))