diff --git a/agent/nix_builder_autoscaler/runtime/ec2.py b/agent/nix_builder_autoscaler/runtime/ec2.py
index 9be3bee..aa5bab5 100644
--- a/agent/nix_builder_autoscaler/runtime/ec2.py
+++ b/agent/nix_builder_autoscaler/runtime/ec2.py
@@ -77,9 +77,6 @@ class EC2Runtime(RuntimeAdapter):
         silently on every failed launch attempt. Never raises; API failures are logged as
         warnings so a transient permissions issue does not prevent startup.
         """
-        if not self._instance_type:
-            return
-
         try:
             target_azs: set[str] = set()
             if self._subnet_ids:
diff --git a/nix/modules/nixos/services/nix-builder-autoscaler.nix b/nix/modules/nixos/services/nix-builder-autoscaler.nix
index 2094b79..e37ce99 100644
--- a/nix/modules/nixos/services/nix-builder-autoscaler.nix
+++ b/nix/modules/nixos/services/nix-builder-autoscaler.nix
@@ -65,7 +65,6 @@ in
 
       instanceType = lib.mkOption {
         type = lib.types.str;
-        default = "";
         description = "EC2 instance type for nix builders. Used for preflight availability validation at startup.";
       };
 
@@ -266,6 +265,10 @@ in
           was not found. Configure package explicitly.
         '';
       }
+      {
+        assertion = cfg.aws.instanceType != "";
+        message = "services.nix-builder-autoscaler.aws.instanceType must be set.";
+      }
       {
         assertion = cfg.aws.launchTemplateIdFile != null;
         message = "services.nix-builder-autoscaler.aws.launchTemplateIdFile must be set.";