diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index b577da7..06efda4 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -1,6 +1,6 @@
 repos:
   - repo: https://github.com/antonbabenko/pre-commit-terraform
-    rev: v1.90.0
+    rev: v1.91.0
     hooks:
       - id: terraform_fmt
       - id: terraform_docs
diff --git a/modules/self-managed-node-group/README.md b/modules/self-managed-node-group/README.md
index 1c025d5..5fb9cd3 100644
--- a/modules/self-managed-node-group/README.md
+++ b/modules/self-managed-node-group/README.md
@@ -165,7 +165,7 @@ module "self_managed_node_group" {
 | <a name="input_network_interfaces"></a> [network\_interfaces](#input\_network\_interfaces) | Customize network interfaces to be attached at instance boot time | `list(any)` | `[]` | no |
 | <a name="input_placement"></a> [placement](#input\_placement) | The placement of the instance | `map(string)` | `{}` | no |
 | <a name="input_placement_group"></a> [placement\_group](#input\_placement\_group) | The name of the placement group into which you'll launch your instances, if any | `string` | `null` | no |
-| <a name="input_platform"></a> [platform](#input\_platform) | [DEPRECATED - use `ami_type` instead. Will be removed in `v21.0`] Identifies the OS platform as `bottlerocket`, `linux` (AL2), `al2023`, or `windows` | `string` | `"linux"` | no |
+| <a name="input_platform"></a> [platform](#input\_platform) | [DEPRECATED - must use `ami_type` instead. Will be removed in `v21.0`] | `string` | `null` | no |
 | <a name="input_post_bootstrap_user_data"></a> [post\_bootstrap\_user\_data](#input\_post\_bootstrap\_user\_data) | User data that is appended to the user data script after of the EKS bootstrap script. Not used when `ami_type` = `BOTTLEROCKET_*` | `string` | `""` | no |
 | <a name="input_pre_bootstrap_user_data"></a> [pre\_bootstrap\_user\_data](#input\_pre\_bootstrap\_user\_data) | User data that is injected into the user data script ahead of the EKS bootstrap script. Not used when `ami_type` = `BOTTLEROCKET_*` | `string` | `""` | no |
 | <a name="input_private_dns_name_options"></a> [private\_dns\_name\_options](#input\_private\_dns\_name\_options) | The options for the instance hostname. The default values are inherited from the subnet | `map(string)` | `{}` | no |
diff --git a/modules/self-managed-node-group/main.tf b/modules/self-managed-node-group/main.tf
index 94844c7..e980453 100644
--- a/modules/self-managed-node-group/main.tf
+++ b/modules/self-managed-node-group/main.tf
@@ -25,9 +25,8 @@ locals {
     AL2023_x86_64_STANDARD     = "al2023"
     AL2023_ARM_64_STANDARD     = "al2023"
   }
-  # Try to use `ami_type` first, but fall back to current, default behavior
-  # TODO - will be removed in v21.0
-  user_data_type = try(local.ami_type_to_user_data_type[var.ami_type], var.platform)
+
+  user_data_type = local.ami_type_to_user_data_type[var.ami_type]
 
   # Map the AMI type to the respective SSM param path
   ami_type_to_ssm_param = {
diff --git a/modules/self-managed-node-group/variables.tf b/modules/self-managed-node-group/variables.tf
index f3b264b..aea1269 100644
--- a/modules/self-managed-node-group/variables.tf
+++ b/modules/self-managed-node-group/variables.tf
@@ -10,10 +10,16 @@ variable "tags" {
   default     = {}
 }
 
+# tflint-ignore: terraform_unused_declarations
 variable "platform" {
-  description = "[DEPRECATED - use `ami_type` instead. Will be removed in `v21.0`] Identifies the OS platform as `bottlerocket`, `linux` (AL2), `al2023`, or `windows`"
+  description = "[DEPRECATED - must use `ami_type` instead. Will be removed in `v21.0`]"
   type        = string
-  default     = "linux"
+  default     = null
+
+  validation {
+    condition     = var.platform == null
+    error_message = "`platform` is no longer valid due to the number of OS choices. Please provide an [`ami_type`](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-eks-nodegroup.html#cfn-eks-nodegroup-amitype) instead."
+  }
 }
 
 ################################################################################
diff --git a/node_groups.tf b/node_groups.tf
index 74a5c56..7228931 100644
--- a/node_groups.tf
+++ b/node_groups.tf
@@ -465,7 +465,7 @@ module "self_managed_node_group" {
   autoscaling_group_tags = try(each.value.autoscaling_group_tags, var.self_managed_node_group_defaults.autoscaling_group_tags, {})
 
   # User data
-  platform = try(each.value.platform, var.self_managed_node_group_defaults.platform, "linux")
+  platform = try(each.value.platform, var.self_managed_node_group_defaults.platform, null)
   # TODO - update this when `var.platform` is removed in v21.0
   ami_type                 = try(each.value.ami_type, var.self_managed_node_group_defaults.ami_type, "AL2_x86_64")
   cluster_endpoint         = try(time_sleep.this[0].triggers["cluster_endpoint"], "")