diff --git a/variables.tf b/variables.tf
index 22fa115..a4d09fd 100644
--- a/variables.tf
+++ b/variables.tf
@@ -28,6 +28,7 @@ variable "vpc_id" {
 variable "subnet_group_name" {
   type        = string
   description = "VPC Id to associate with Redis ElastiCache."
+  default     = null
 }
 
 variable "source_cidr_blocks" {