diff --git a/pkg/instancegroups/rollingupdate_test.go b/pkg/instancegroups/rollingupdate_test.go
index 9ea9f76f89..baaee45d4b 100644
--- a/pkg/instancegroups/rollingupdate_test.go
+++ b/pkg/instancegroups/rollingupdate_test.go
@@ -136,7 +136,7 @@ func makeGroup(groups map[string]*cloudinstances.CloudInstanceGroup, k8sClient k
 
 	var instanceIds []*string
 	for i := 0; i < count; i++ {
-		id := name + string('a'+i)
+		id := name + string(rune('a'+i))
 		var node *v1.Node
 		if role != kopsapi.InstanceGroupRoleBastion {
 			node = &v1.Node{
diff --git a/upup/pkg/fi/cloudup/awsup/machine_types.go b/upup/pkg/fi/cloudup/awsup/machine_types.go
index 63d89aed78..abc1fe2e65 100644
--- a/upup/pkg/fi/cloudup/awsup/machine_types.go
+++ b/upup/pkg/fi/cloudup/awsup/machine_types.go
@@ -58,7 +58,7 @@ func (m *AWSMachineTypeInfo) EphemeralDevices() []*EphemeralDevice {
 			// TODO: What drive letters do we use?
 			klog.Fatalf("ephemeral devices for > 20 not yet implemented")
 		}
-		d.DeviceName = "/dev/sd" + string('c'+i)
+		d.DeviceName = fmt.Sprintf("/dev/sd%c", 'c'+i)
 		d.VirtualName = fmt.Sprintf("ephemeral%d", i)
 
 		disks = append(disks, d)