diff --git a/vpc-transit-gateway-association/routing.tf b/vpc-transit-gateway-association/routing.tf
index d3a497c..241d333 100644
--- a/vpc-transit-gateway-association/routing.tf
+++ b/vpc-transit-gateway-association/routing.tf
@@ -33,7 +33,7 @@ data "aws_ec2_managed_prefix_lists" "tgw_ipv4" {
 }
 
 data "aws_ec2_managed_prefix_list" "tgw_ipv4" {
-  for_each = data.aws_ec2_managed_prefix_lists.tgw_ipv4.ids
+  for_each = toset(data.aws_ec2_managed_prefix_lists.tgw_ipv4.ids)
   id       = each.key
 }