diff --git a/nacl-rules/main.tf b/nacl-rules/main.tf
index 7af883e..7332493 100644
--- a/nacl-rules/main.tf
+++ b/nacl-rules/main.tf
@@ -106,7 +106,7 @@ locals {
   r4_out = [for v in local.r3_out : v.rule_number]
   r5_in  = length(local.r4_in) > 0 ? max(local.r4_in...) : null
   r5_out = length(local.r4_out) > 0 ? max(local.r4_out...) : null
-  r5     = max(local.r5_in, local.r5_out)
+  r5     = local.r5_in != null && local.r5_out != null ? max(local.r5_in, local.r5_out) : null
 }
 
 resource "aws_network_acl_rule" "in" {