diff --git a/rds-postgres/main.tf b/rds-postgres/main.tf
index bfee84d..b1f8443 100644
--- a/rds-postgres/main.tf
+++ b/rds-postgres/main.tf
@@ -68,7 +68,7 @@ resource "aws_security_group" "this_security_group" {
 
   # ingress with prefix lists
   dynamic "ingress" {
-    for_each = length(var.ingress_prefix_list_names) > 0 ? local.port_map["external"] : {}
+    for_each = length(var.ingress_prefix_list_names) > 0 ? local.port_map["external"] : toset([])
     iterator = p
     content {
       description     = "${local.short_description}: ${p.value["description"]}"
@@ -132,7 +132,7 @@ resource "aws_security_group" "this_security_group" {
 
   # egress with prefix lists
   dynamic "egress" {
-    for_each = length(var.egress_prefix_list_names) > 0 ? local.port_map["external"] : {}
+    for_each = length(var.egress_prefix_list_names) > 0 ? local.port_map["external"] : toset([])
     iterator = p
     content {
       description     = "${local.short_description}: ${local.egress_sg_names[sg]}"