diff --git a/cmd/kubeadm/app/apis/kubeadm/validation/validation.go b/cmd/kubeadm/app/apis/kubeadm/validation/validation.go
index 3220b41aab4..c765ca301ba 100644
--- a/cmd/kubeadm/app/apis/kubeadm/validation/validation.go
+++ b/cmd/kubeadm/app/apis/kubeadm/validation/validation.go
@@ -53,6 +53,7 @@ var cloudproviders = []string{
 	"azure",
 	"cloudstack",
 	"gce",
+	"external", // Support for out-of-tree cloud providers
 	"openstack",
 	"ovirt",
 	"photon",