diff --git a/variables.tf b/variables.tf
index cdfdd6d6..6eae4716 100644
--- a/variables.tf
+++ b/variables.tf
@@ -16,6 +16,7 @@ variable "subdomain" {
   description = "Subdomain of the cluster. Value used for the ingress' URL of the application."
   type        = string
   default     = "apps"
+  nullable    = false
 }
 
 variable "argocd_project" {