diff --git a/cmd/k8s-operator/proxygroup.go b/cmd/k8s-operator/proxygroup.go
index 39b7ccc01..60f470fc2 100644
--- a/cmd/k8s-operator/proxygroup.go
+++ b/cmd/k8s-operator/proxygroup.go
@@ -479,7 +479,7 @@ func pgTailscaledConfig(pg *tsapi.ProxyGroup, class *tsapi.ProxyClass, idx int32
 	}
 
 	if pg.Spec.HostnamePrefix != "" {
-		conf.Hostname = ptr.To(fmt.Sprintf("%s%d", pg.Spec.HostnamePrefix, idx))
+		conf.Hostname = ptr.To(fmt.Sprintf("%s-%d", pg.Spec.HostnamePrefix, idx))
 	}
 
 	if shouldAcceptRoutes(class) {