diff --git a/cer.install b/cer.install
index 0fb6a44..f66d9e0 100644
--- a/cer.install
+++ b/cer.install
@@ -16,6 +16,7 @@ function cer_schema() {
     'fields' => array(
       'pid' => array(
         'type' => 'serial',
+        'not null' => TRUE,
       ),
       'identifier' => array(
         'type' => 'text',