diff --git a/domain.install b/domain.install
index 890fba4..6c14895 100644
--- a/domain.install
+++ b/domain.install
@@ -66,7 +66,7 @@ function domain_schema() {
   $schema['domain_export'] = array(
     'description' => 'Stores canonical machine names for domains.',
     'fields' => array(
-      'domain_id' => array('type' => 'serial', 'description' => 'Domain id. Automatic master key.'),
+      'domain_id' => array('type' => 'serial', 'not null' => TRUE, 'description' => 'Domain id. Automatic master key.'),
       'machine_name' => array('type' => 'varchar', 'length' => '255', 'not null' => TRUE, 'default' => '', 'description' => 'The machine name for this domain.')),
     'primary key' => array('machine_name'),
     'indexes' => array(