diff --git a/routing/main.tf b/routing/main.tf
index 563d111..8182ddc 100644
--- a/routing/main.tf
+++ b/routing/main.tf
@@ -32,7 +32,7 @@ locals {
     "boc:created_by"        = "terraform"
   }
 
-  availability_zones = length(var.availability_zones) != 0 ? var.availability_zones : data.aws_availability_zones.zones.names
+  availability_zones = length(var.availability_zones) != 0 ? toset(var.availability_zones) : toset(data.aws_availability_zones.zones.names)
   az_count           = length(local.availability_zones)
   az_count_list      = range(local.az_count)
   az_list            = toset(local.availability_zones)