diff --git a/promql/engine.go b/promql/engine.go
index f1904ec4ba..6ce0ebcb29 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -1604,7 +1604,7 @@ func (ev *evaluator) eval(ctx context.Context, expr parser.Expr) (parser.Value,
 		if e.Op == parser.COUNT_VALUES {
 			valueLabel := param.(*parser.StringLiteral)
 			if !model.LabelName(valueLabel.Val).IsValid() {
-				ev.errorf("invalid label name %q", valueLabel)
+				ev.errorf("invalid label name %s", valueLabel)
 			}
 			if !e.Without {
 				sortedGrouping = append(sortedGrouping, valueLabel.Val)
diff --git a/promql/promqltest/testdata/aggregators.test b/promql/promqltest/testdata/aggregators.test
index 7479d2c3d3..1e3ab79a35 100644
--- a/promql/promqltest/testdata/aggregators.test
+++ b/promql/promqltest/testdata/aggregators.test
@@ -430,6 +430,10 @@ eval instant at 1m count_values by (job, group)("job", version)
 	{job="7", group="canary"} 2
     {job="{count:20, sum:10, [-2,-1):2, [-1,-0.5):1, [-0.001,0.001]:2, (0.5,1]:1, (1,2]:2}", group="canary"} 2
 
+# Test an invalid label value.
+eval_fail instant at 0 count_values("a\xc5z", version)
+  expected_fail_message invalid label name "a\xc5z"
+
 # Tests for quantile.
 clear