diff --git a/rds-postgres/ports.tf b/rds-postgres/ports.tf
index 5737c50..50dc4ed 100644
--- a/rds-postgres/ports.tf
+++ b/rds-postgres/ports.tf
@@ -18,8 +18,8 @@ locals {
   ]
 
   # these are ignored
-  ingress_networks = var.ingress_networks
-  egress_networks  = var.egress_networks
+  ingress_networks = var.ingress_networks == null ? [] : var.ingress_networks
+  egress_networks  = var.egress_networks == null ? [] : var.egress_networks
 
   # these are ignored
   ingress_sg = var.ingress_security_groups