diff --git a/terraform/aws/control-plane.tf b/terraform/aws/control-plane.tf
index 618f24e..7aa0071 100644
--- a/terraform/aws/control-plane.tf
+++ b/terraform/aws/control-plane.tf
@@ -40,7 +40,7 @@ module "asg" {
 
   # Auto scaling group
   asg_name                  = "${local.service}-leader-asg"
-  vpc_zone_identifier       = ["${data.aws_subnet_ids.private.ids}"]
+  vpc_zone_identifier       = [join(",", data.aws_subnet_ids.private.ids)]
   health_check_type         = "EC2"
   min_size                  = 1
   max_size                  = 1