diff --git a/variables.tf b/variables.tf
index 44b8b73..60d7624 100644
--- a/variables.tf
+++ b/variables.tf
@@ -38,6 +38,11 @@ variable "environment" {
 variable "openresty_source_type" {
   description = "OpenResty config source: 'bind_mount', 'local_build', or 'git_clone'."
   type        = string
+
+  validation {
+    condition     = contains(["bind_mount", "local_build", "git_clone"], var.openresty_source_type)
+    error_message = "openresty_source_type must be 'bind_mount', 'local_build', or 'git_clone'."
+  }
 }
 
 variable "openresty_image" {