diff --git a/models/CustomFieldModel.php b/models/CustomFieldModel.php
index 4ebb3c0..cc80aa2 100644
--- a/models/CustomFieldModel.php
+++ b/models/CustomFieldModel.php
@@ -8,6 +8,9 @@ class CustomFieldModel
 {
     private $conn;
 
+    // Must match custom_field_definitions.field_type's enum() in the schema.
+    private const ALLOWED_FIELD_TYPES = ['text', 'textarea', 'select', 'checkbox', 'date', 'number'];
+
     public function __construct($conn)
     {
         $this->conn = $conn;
@@ -87,6 +90,10 @@ class CustomFieldModel
      */
     public function createDefinition($data)
     {
+        if (!in_array($data['field_type'] ?? '', self::ALLOWED_FIELD_TYPES, true)) {
+            return ['success' => false, 'error' => 'Invalid field_type'];
+        }
+
         $options = null;
         if (isset($data['field_options']) && !empty($data['field_options'])) {
             $options = json_encode($data['field_options']);
@@ -129,6 +136,10 @@ class CustomFieldModel
      */
     public function updateDefinition($fieldId, $data)
     {
+        if (!in_array($data['field_type'] ?? '', self::ALLOWED_FIELD_TYPES, true)) {
+            return ['success' => false, 'error' => 'Invalid field_type'];
+        }
+
         $options = null;
         if (isset($data['field_options']) && !empty($data['field_options'])) {
             $options = json_encode($data['field_options']);